A communication device for RNTI allocation comprises a processing means, for executing a program, and a storage unit, coupled to the processing means, for storing the program which instructs the processing means to perform the steps: connecting to a first base station, receiving a first RNTI assigned by the first base station, receiving a RRC message including a second RNTI for communication with a second base station from the first base station, transmitting a RRC message response to the first base station, performing communication with the first base station with the first RNTI and communication with the second base station with the second RNTI, transmitting a random access preamble to the second base station, receiving a random access response from the second base station, transmitting a scheduled transmission containing the second RNTI to the second base station, and receiving a contention resolution message from the second base station.