how to open disk defrag in c#.net

It works fine on my machine (Windows 7) where dfrgui.exe is located in the windows system directory: c:\windows\system32 though that's in my path environment variable so I don't need to specify it.

See if it will work if you specify a full path:

System.Diagnostics.Process.Start(@"c:\windows\system32\dfrgui.exe");

it is not work.

Try:

System.Diagnostics.Process.Start("dfrgui.exe");