today,l learned have to get transparent bitmap in .Net.
It is very convenient and softly.
as below:
Dim pBmp As System.Drawing.Bitmap = System.Drawing.Image.FromHbitmap(pCmd.Bitmap)
pBmp.MakeTransparent(pBmp.GetPixel(0, 0))
pBtn.Image = pBmp
note:
pCmd is ESRI.SytemUI.ICommand
pBtn is System.Windows.Forms.Button