diff --git a/doc/newsfragments/2991_changed.auto_parts_refine.rst b/doc/newsfragments/2991_changed.auto_parts_refine.rst
new file mode 100644
index 000000000..f3d7e7b6e
--- /dev/null
+++ b/doc/newsfragments/2991_changed.auto_parts_refine.rst
@@ -0,0 +1,2 @@
+Refine Auto-Part feature to handle corner cases when zero/negative/overly large number of parts are calculated.
+
diff --git a/testplan/runnable/base.py b/testplan/runnable/base.py
index fbfb28ee2..1a7df2720 100644
--- a/testplan/runnable/base.py
+++ b/testplan/runnable/base.py
@@ -592,19 +592,46 @@ def _get_tasks(
                     )
                     num_of_parts = 1
                 else:
-                    num_of_parts = math.ceil(
+                    # the setup time shall take no more than 50% of runtime
+                    cap = math.ceil(
                         time_info["execution_time"]
-                        / (
-                            self.cfg.auto_part_runtime_limit
-                            - time_info["setup_time"]
-                        )
+                        / self.cfg.auto_part_runtime_limit
+                        * 2
                     )
+                    formula = f"""
+    num_of_parts = math.ceil(
+        time_info["execution_time"] {time_info["execution_time"]}
+        / (
+            self.cfg.auto_part_runtime_limit {self.cfg.auto_part_runtime_limit}
+            - time_info["setup_time"] {time_info["setup_time"]}
+        )
+    )
+"""
+                    try:
+                        num_of_parts = math.ceil(
+                            time_info["execution_time"]
+                            / (
+                                self.cfg.auto_part_runtime_limit
+                                - time_info["setup_time"]
+                            )
+                        )
+                    except ZeroDivisionError:
+                        self.logger.error(
+                            f"ZeroDivisionError occurred when calculating num_of_parts for {uid}, set to 1. {formula}"
+                        )
+                        num_of_parts = 1
+
                     if num_of_parts < 1:
+                        self.logger.error(
+                            f"Calculated num_of_parts for {uid} is {num_of_parts}, set to 1. {formula}"
+                        )
                         num_of_parts = 1
+
+                    if num_of_parts > cap:
                         self.logger.error(
-                            f"Calculated num_of_parts for {uid} is {num_of_parts},"
-                            " check the input runtime_data and auto_part_runtime_limit"
+                            f"Calculated num_of_parts for {uid} is {num_of_parts} > cap {cap}, set to {cap}. {formula}"
                         )
+                        num_of_parts = cap
             if "weight" not in _task_arguments:
                 _task_arguments["weight"] = (
                     math.ceil(
diff --git a/tests/functional/testplan/runners/pools/test_auto_part.py b/tests/functional/testplan/runners/pools/test_auto_part.py
index 6d244fe59..f1f614033 100755
--- a/tests/functional/testplan/runners/pools/test_auto_part.py
+++ b/tests/functional/testplan/runners/pools/test_auto_part.py
@@ -97,3 +97,63 @@ def test_auto_weight_discover():
             assert task.weight == 140
         mockplan.run()
         assert pool.size == 1
+
+
+def test_auto_parts_zero_neg_parts():
+    with tempfile.TemporaryDirectory() as runpath:
+        mockplan = TestplanMock(
+            "plan",
+            runpath=runpath,
+            merge_scheduled_parts=True,
+            auto_part_runtime_limit=45,
+            plan_runtime_target=200,
+            runtime_data={
+                "Proj1-suite": {
+                    "execution_time": 50,
+                    "setup_time": 50,  # setup_time > runtime_limit -> negtive num_of_parts
+                }
+            },
+        )
+        pool = ProcessPool(name="MyPool", size="auto")
+        mockplan.add_resource(pool)
+        current_folder = os.path.dirname(os.path.realpath(__file__))
+        mockplan.schedule_all(
+            path=f"{current_folder}/discover_tasks",
+            name_pattern=r".*auto_parts_tasks\.py$",
+            resource="MyPool",
+        )
+        assert len(pool.added_items) == 1
+        for task in pool.added_items.values():
+            assert task.weight == 100
+        mockplan.run()
+        assert pool.size == 1
+
+
+def test_auto_parts_cap_parts():
+    with tempfile.TemporaryDirectory() as runpath:
+        mockplan = TestplanMock(
+            "plan",
+            runpath=runpath,
+            merge_scheduled_parts=True,
+            auto_part_runtime_limit=45,
+            plan_runtime_target=200,
+            runtime_data={
+                "Proj1-suite": {
+                    "execution_time": 60,
+                    "setup_time": 44,
+                }
+            },
+        )
+        pool = ProcessPool(name="MyPool", size="auto")
+        mockplan.add_resource(pool)
+        current_folder = os.path.dirname(os.path.realpath(__file__))
+        mockplan.schedule_all(
+            path=f"{current_folder}/discover_tasks",
+            name_pattern=r".*auto_parts_tasks\.py$",
+            resource="MyPool",
+        )
+        assert len(pool.added_items) == 3
+        for task in pool.added_items.values():
+            assert task.weight == 64
+        mockplan.run()
+        assert pool.size == 1