I found the problem, I was passing the command button reference instead of the form reference
DO FORM ChildForm WITH THIS.PARENT
every thing working fine now, thanks for help

>You need to pass the reference.
>
>Parent form:

DO FORM ChildForm WITH THIS

Child form Init:

LPARAMETERS toParent AS Form
>
>THIS.MyParent = toParent
>
>...
>
>THIS.MyParent.Caption = "New caption"

HTH
>
>>How can I access the properties of Parent Form from Child Form ?