diff --git a/src/kirin/dialects/ilist/rewrite/flatten_add.py b/src/kirin/dialects/ilist/rewrite/flatten_add.py
index 0792e5136..f39b13390 100644
--- a/src/kirin/dialects/ilist/rewrite/flatten_add.py
+++ b/src/kirin/dialects/ilist/rewrite/flatten_add.py
@@ -18,7 +18,18 @@ def rewrite_Statement(self, node: ir.Statement) -> RewriteResult:
         new_data = ()
 
         # lhs:
-        if isinstance((lhs := node.lhs).owner, ilist.New):
+        lhs = node.lhs
+        rhs = node.rhs
+
+        if (
+            (lhs_parent := lhs.owner.parent) is None
+            or (rhs_parent := rhs.owner.parent) is None
+            or lhs_parent is not rhs_parent
+        ):
+            # do not flatten across different blocks/regions
+            return RewriteResult()
+
+        if isinstance(lhs.owner, ilist.New):
             new_data += lhs.owner.values
         elif (
             not isinstance(const_lhs := lhs.hints.get("const"), const.Value)
@@ -27,7 +38,7 @@ def rewrite_Statement(self, node: ir.Statement) -> RewriteResult:
             return RewriteResult()
 
         # rhs:
-        if isinstance((rhs := node.rhs).owner, ilist.New):
+        if isinstance(rhs.owner, ilist.New):
             new_data += rhs.owner.values
         elif (
             not isinstance(const_rhs := rhs.hints.get("const"), const.Value)
diff --git a/test/dialects/test_ilist.py b/test/dialects/test_ilist.py
index 7a860e4e3..68b34ba4e 100644
--- a/test/dialects/test_ilist.py
+++ b/test/dialects/test_ilist.py
@@ -2,7 +2,7 @@
 
 from kirin import ir, types, rewrite
 from kirin.passes import aggressive
-from kirin.prelude import basic_no_opt, python_basic
+from kirin.prelude import structural, basic_no_opt, python_basic
 from kirin.analysis import const
 from kirin.dialects import py, func, ilist, lowering
 from kirin.passes.typeinfer import TypeInfer
@@ -328,6 +328,46 @@ def test_ilist_flatten_add_both_new():
     assert test_block.is_structurally_equal(expected_block)
 
 
+def test_region_boundary_structural():
+
+    # Do not optimize across region boundary like if-else or basic blocks
+    @structural
+    def test_impl(n: int):
+        a = ilist.IList([])
+
+        if n > 0:
+            a = a + [n]
+
+        return a
+
+    expected_impl = test_impl.similar()
+    test_impl.print(hint="const")
+    rule = rewrite.Walk(ilist.rewrite.FlattenAdd())
+    rule.rewrite(test_impl.code)
+
+    assert test_impl.code.is_structurally_equal(expected_impl.code)
+
+
+def test_region_boundary():
+
+    # Do not optimize across region boundary like if-else or basic blocks
+    @basic_no_opt
+    def test_impl(n: int):
+        a = ilist.IList([])
+
+        if n > 0:
+            a = a + [n]
+
+        return a
+
+    expected_impl = test_impl.similar()
+    test_impl.print(hint="const")
+    rule = rewrite.Walk(ilist.rewrite.FlattenAdd())
+    rule.rewrite(test_impl.code)
+
+    assert test_impl.code.is_structurally_equal(expected_impl.code)
+
+
 def test_ilist_constprop():
     from kirin.analysis import const