>How can I know the existance of PEM eg. I like to know whether the property named pDate already defined.

You can use PEMSTATUS(,, 5)

loObj = CREATEOBJECT("Empty")
ADDPROPERTY(loObj, "Prop1", "Some Value")
? PEMSTATUS(loObj, "Prop1", 5)		&& .T.
? PEMSTATUS(loObj, "Prop2", 5)		&& .F.