A method and apparatus for enhancing a handover procedure for supporting conditional handover in a wireless communication system. A source base station (e.g. source gNB/eNB) receives a message informing that a wireless device is handed over from the source base station to a target base station (e.g. target gNB/eNB). The target base station is one of candidate target base stations in the conditional handover procedure. After receiving the message, the source base station transmits a sequence number (SN) status transfer message to the target base station.