Hello Nafiseh, Thank you for sharing this code snippet. It will make each of the RadDiagramShapes in RTL mode. However, RadDiagram is not designed to support RightToLeft mode. Hence, we can't guarantee that this code snippet will work for all scenarios.
You can used this code for solved this problem: radDiagram1.Items.Where(r=>r.GetType() == typeof(RadDiagramShape)).ForEach(r=>r.RightToLeft = True)