A method and apparatus are disclosed for adding serving cells in a wireless communication system. The method includes a UE (User Equipment) is served by a first cell controlled by a first eNB (evolved Node B), wherein multiple radio bearers (RBs) for the UE are allocated to the first eNB and there is a MAC (Medium Access Control) entity in the UE for supporting the first cell. The method also includes the UE receives a RRC (Radio Resource Control) message for adding a second cell controlled by a second eNB to the UE, wherein a RB among the multiple RBs is relocated to the second eNB according to information included in the RRC message. The method further includes the UE re-establishes a RLC (Radio Link Control) entity corresponding to the RB and does not reset the MAC entity in response to the relocation of the RB.