diff --git a/src/Add-on/Extends.ts b/src/Add-on/Extends.ts
index ee0344e26..5ffd5c849 100644
--- a/src/Add-on/Extends.ts
+++ b/src/Add-on/Extends.ts
@@ -9,6 +9,7 @@ import { Command } from '../Editor/CommandMachine';
 import { JigUtils } from '../Editor/JigUtils';
 import { PromptSsgetResult, PromptStatus } from '../Editor/PromptResult';
 import { SelectBox, SelectType } from '../Editor/SelectBox';
+import { SelectLine } from '../Editor/SelectLine';
 import { SelectPick } from '../Editor/SelectPick';
 import { SelectSet } from '../Editor/SelectSet';
 import { AsVector3 } from '../Geometry/GeUtils';
@@ -83,6 +84,10 @@ export class Command_Extend implements Command
             {
                 selectCenterPt = set._PickPointVcs.clone();
             }
+            else if (set instanceof SelectLine)
+            {
+                selectCenterPt = AsVector3(set.Center);
+            }
             app.Viewer.ScreenToWorld(selectCenterPt);
 
             for (let cu of set.SelectEntityList as Curve[])
diff --git a/src/Editor/SelectLine.ts b/src/Editor/SelectLine.ts
index 3e836340e..55411a51e 100644
--- a/src/Editor/SelectLine.ts
+++ b/src/Editor/SelectLine.ts
@@ -1,5 +1,6 @@
 import { Matrix4, Object3D, OrthographicCamera, PerspectiveCamera, Vector2, Vector3 } from 'three';
 import { doIntersect } from "../Geometry/DoIntersect";
+import { midPoint2 } from '../Geometry/GeUtils';
 import { IViewer } from '../GraphicsSystem/IView';
 import { SelectSetBase } from './SelectBase';
 
@@ -23,6 +24,11 @@ export class SelectLine extends SelectSetBase
         }
     }
 
+    get Center()
+    {
+        return midPoint2(this._StartPt, this._EndPt);
+    }
+
     IntersectLine(p1: Vector3, p2: Vector3): boolean
     {
         return doIntersect(p1, p2, this._StartPt, this._EndPt);