I solved my problem by using this line: if(tmp=="PassDialog") { //QLabel* lbl = f->findChild<QLabel*>("lblRFID"); MyDialog *tempDialog = qobject_cast<MyDialog*>(f); tempDialog->AcceptCard(output); // emit recRFID(output); return; } and clean that connection and comment the emit code. now its worked. But I don't know why that code has error.