A method for performing a handover by a user equipment from a serving base station to a target base station includes transmitting a random access preamble to the target base station, receiving a random access response in response to the random access preamble; after receiving the random access response, transmitting a handover confirm message for indicating completion of the handover between the user equipment and the target base station, and transmitting a packet data sequence number report message indicating packet data received from the serving base station in a process of performing the handover. Repeated transmission of downlink data or uplink data can be avoided during a handover procedure.