A method for buffer status report (BSR) in dual connectivity for a communication device in a wireless communication system is disclosed. The method comprises connecting to at least two base stations including a first base station and a second base station in the wireless communication system, receiving a BSR configuration from the first base station, wherein the BSR configuration indicates a calculation scheme for a BSR, and calculating buffer sizes indicated in a first BSR and a second BSR respectively for the first base station and the second base station according to the calculation scheme in the received BSR configuration, wherein at least a first logical channel of a first logical channel group reported by the first BSR and at least a second logical channel of a second logical channel group reported by the second BSR are mapped to the same split radio bearer.