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 configuring a second RNTI, a first random access preamble and a random access channel, RACH, resource location for configuring communication with a second base station of the wireless communication system from the first base station; and wherein the second RNTI, the first random access preamble and the RACH resource location are 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 the first random access preamble in the RACH resource location to the second base station; the communication device receiving a first random access response from the second base station in response to the first random access preamble, wherein the first random access response contains a random access preamble identifier corresponding to the first random access preamble, a timing advance command and an uplink grant; and the communication device applying the timing advance command for uplink time alignment with 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 configuring a second RNTI, a first random access preamble and a random access channel, RACH, resource location for configuring communication with a second base station of the wireless communication system from the first base station; and wherein the second RNTI, the first random access preamble and the RACH resource location are 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 the first random access preamble in the RACH resource location to the second base station; the communication device receiving a first random access response from the second base station in response to the first random access preamble, wherein the first random access response contains a random access preamble identifier corresponding to the first random access preamble, a timing advance command and an uplink grant; and the communication device applying the timing advance command for uplink time alignment with the second base station.