A method of radio network temporary identifier, RNTI, allocation to enable a dual connectivity for a communication device in a wireless communication system, comprising: the communication device connecting to a first base station of the wireless communication system; the communication device receiving a first RNTI assigned by the first base station for communication with the first base station; the communication device receiving a radio resource control, RRC, message including a second RNTI for configuring communication with a second base station of the wireless communication system from the first base station, wherein the second RNTI is assigned by the second base station; the communication device transmitting a RRC message response to the first base station in response to the RRC message (404, 406); the communication device performing communication with the first base station with the first RNTI and performing communication with the second base station with the second RNTI, in dual connectivity; the communication device transmitting a random access preamble in a random access channel, RACH, resource location to the second base station, wherein the random access preamble is selected by the communication device from a set of preambles broadcasted by the second base station in system information or configured in the RRC message, and the RACH resource location is configured in the RRC message or broadcasted by the second base station in the system information; the communication device receiving a random access response from the second base station in response to the random access preamble, wherein the random access response contains a random access preamble identifier corresponding to the random access preamble, a timing advance command and an uplink grant; the communication device applying the timing advance command for uplink time alignment with the second base station; the communication device transmitting a scheduled transmission containing the second RNTI to the second base station according to the uplink grant; and the communication device receiving a contention resolution message containing the second RNTI from the second base station. the communication device connecting to a first base station of the wireless communication system; the communication device receiving a first RNTI assigned by the first base station for communication with the first base station; the communication device receiving a radio resource control, RRC, message including a second RNTI for configuring communication with a second base station of the wireless communication system from the first base station, wherein the second RNTI is assigned by the second base station; the communication device transmitting a RRC message response to the first base station in response to the RRC message (404, 406); the communication device performing communication with the first base station with the first RNTI and performing communication with the second base station with the second RNTI, in dual connectivity; the communication device transmitting a random access preamble in a random access channel, RACH, resource location to the second base station, wherein the random access preamble is selected by the communication device from a set of preambles broadcasted by the second base station in system information or configured in the RRC message, and the RACH resource location is configured in the RRC message or broadcasted by the second base station in the system information; the communication device receiving a random access response from the second base station in response to the random access preamble, wherein the random access response contains a random access preamble identifier corresponding to the random access preamble, a timing advance command and an uplink grant; the communication device applying the timing advance command for uplink time alignment with the second base station; the communication device transmitting a scheduled transmission containing the second RNTI to the second base station according to the uplink grant; and the communication device receiving a contention resolution message containing the second RNTI from the second base station.