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 is disclosed. The method is utilized in the first base station and comprises transmitting a request message to the second base station, for requesting a cell addition for the communication device; receiving a response message comprising system information of a cell of the second base station from the second base station in response to the request message; and transmitting a first radio resource control (RRC) message comprising the system information to the communication device, after receiving the response message.