A method for handling a reactivation request from the network for an MA PDU session with session and service continuity (SSC) mode 3 is proposed. When UE receives a PDU SESSION MODIFICATION COMMAND message which includes 5GSM cause #39 “reactivation requested” for an existing MA PDU session, UE handles the network-requested PDU session modification procedure, and UE re-initiates the UE-requested PDU session establishment procedure with a new MA PDU session ID. UE determines how to proceed with a PDU session reestablishment procedure for the new MA PDU session based on different scenarios of UE registration and user plane resources of the existing MA PDU session, and/or also based on whether the PDU SESSION MODIFICATION COMMAND message is sent over 3GPP access type or non-3GPP access type.