A communication device of handling a detach procedure comprises a storage unit for storing instructions and a processing means coupled to the storage unit. The processing means is configured to execute the instructions stored in the storage unit. The instructions comprise attaching to a network in a Network Mode of Operation I (NMO I) for a circuit switched (CS) domain and a packet switched (PS) domain; transmitting a first General Packet Radio Service (GPRS) mobility management (GMM) message to the network; receiving a second GMM message comprising a GMM cause indicating an implicit detach from the network; and performing an International Mobile Subscriber Identity (IMSI) detach for the PS domain but not for the CS domain, when a Radio Resource (RR) connection exists.