diff --git a/.chronus/changes/types_order-2024-9-31-14-3-9.md b/.chronus/changes/types_order-2024-9-31-14-3-9.md
new file mode 100644
index 0000000000..9cdc6f9010
--- /dev/null
+++ b/.chronus/changes/types_order-2024-9-31-14-3-9.md
@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@azure-tools/typespec-client-generator-core"
+---
+
+use array instead of set to make the types ordered by typespec definition
\ No newline at end of file
diff --git a/packages/typespec-client-generator-core/src/types.ts b/packages/typespec-client-generator-core/src/types.ts
index 22224e9904..72a7796aae 100644
--- a/packages/typespec-client-generator-core/src/types.ts
+++ b/packages/typespec-client-generator-core/src/types.ts
@@ -1752,15 +1752,17 @@ function filterOutTypes(
   context: TCGCContext,
   filter: number,
 ): (SdkModelType | SdkEnumType | SdkUnionType | SdkNullableType)[] {
-  const result = new Set<SdkModelType | SdkEnumType | SdkUnionType | SdkNullableType>();
+  const result = new Array<SdkModelType | SdkEnumType | SdkUnionType | SdkNullableType>();
   for (const sdkType of context.referencedTypeMap?.values() ?? []) {
     // filter models with unexpected usage
     if ((sdkType.usage & filter) === 0) {
       continue;
     }
-    result.add(sdkType);
+    if (!result.includes(sdkType)) {
+      result.push(sdkType);
+    }
   }
-  return [...result];
+  return result;
 }
 
 export function getAllModelsWithDiagnostics(