Use this tool to round a corner created by 2 intersecting lines. 
Note: You can only fillet elements that are created with the Line (this includes sides of polygons) or Arc sketch options. 
- Select a tool that allows for an arc. 
  For example, click Annotate tab Detail panel Detail panel  (Detail Line). (Detail Line).
 
-  Click Modify | Place Lines tab (or respective Modify | Place <Element> tab or Modify | Create <Element> tab) Draw panel Draw panel  (Fillet Arc), and place the cursor in the drawing area. (Fillet Arc), and place the cursor in the drawing area.
- Select the first element to fillet. 
- Select the second element to fillet. 
- Move the cursor and click to define the fillet arc. 
 
After you place the fillet, 
  Revit automatically trims the elements to the fillet.