先用findwindow得到你要查的窗口的hwnd，然后用sendmessge yourform.hwnd,wm_close,0

private button1_click()
dim tmp as long
tmp=findwindow(vbnullstring,"程序的窗口名VB中FORM的NAME属性值")
if tmp > 0 then
sendmessage tmp,wm_close,0
else
msgbox "Sorry!Don＇t find formname"
end if
end sub