A method of handling a cell addition for dual connectivity in a communication system comprising a communication device, a first base station and a second base station, the communication device being connected to the first base station, the method comprising: the communication device receiving a first radio resource control, RRC, message comprising system information of a cell of the second base station from the first base station; the communication device transmitting a RRC response message to the first base station in response to the first RRC message; the communication device communicating with the cell of the second base station according to the system information; the first base station transmitting a request message to the second base station, for requesting a cell addition of the cell of the second base station for the communication device (302, 402); the first base station receiving a response message comprising the system information from the second base station in response to the request message (304, 404); and the first base station transmitting the first RRC message after receiving the response message (306, 406); wherein the system information comprises an uplink configuration of the cell of the second base station and/or a downlink configuration of the cell of the second base station; wherein the uplink configuration comprises at least one of an uplink carrier frequency, an uplink bandwidth, additional spectrum emission, a parameter p-Max, an uplink power control, a sounding uplink reference signal configuration, an uplink cyclic prefix length, a physical random access channel configuration, a physical uplink shared channel configuration, a random access channel configuration and a time-division duplexing, TDD, configuration; and wherein the downlink configuration comprises at least one of at least one of a downlink bandwidth, antenna information, a multicast-broadcast single-frequency network subframe configuration, a physical HARQ indicator channel configuration, a physical downlink shared channel configuration and a time-division duplexing, TDD, configuration. the communication device receiving a first radio resource control, RRC, message comprising system information of a cell of the second base station from the first base station; the communication device transmitting a RRC response message to the first base station in response to the first RRC message; the communication device communicating with the cell of the second base station according to the system information; the first base station transmitting a request message to the second base station, for requesting a cell addition of the cell of the second base station for the communication device (302, 402); the first base station receiving a response message comprising the system information from the second base station in response to the request message (304, 404); and the first base station transmitting the first RRC message after receiving the response message (306, 406); wherein the system information comprises an uplink configuration of the cell of the second base station and/or a downlink configuration of the cell of the second base station; wherein the uplink configuration comprises at least one of an uplink carrier frequency, an uplink bandwidth, additional spectrum emission, a parameter p-Max, an uplink power control, a sounding uplink reference signal configuration, an uplink cyclic prefix length, a physical random access channel configuration, a physical uplink shared channel configuration, a random access channel configuration and a time-division duplexing, TDD, configuration; and wherein the downlink configuration comprises at least one of at least one of a downlink bandwidth, antenna information, a multicast-broadcast single-frequency network subframe configuration, a physical HARQ indicator channel configuration, a physical downlink shared channel configuration and a time-division duplexing, TDD, configuration.