A method for maintaining MBS session when UE performs intersystem change is provided. UE maintains a PDU session in 5G network. UE then joins an MBS session for receiving MBS service over the associated PDU session. Later on, UE performs intersystem change from 5G to 2/3/4G. The PDU session is then released or transferred to a PDN connection. From UE perspective, UE determines that the released or transferred PDU session is associated with the MBS session, and therefore locally leaves the MBS session. From NW perspective, NW determines that the released or transferred PDU session is associated with the MBS session, and therefore considers the UE is removed from the MBS session.