A data transmission method in a mobile communication system is disclosed. The present invention is suitable for retaining continuity of a data service in case that a mobile subscriber station carries out handover. The present invention includes the steps of receiving a message information (S41) indicating a handover completion of a mobile subscriber station from the second base station, releasing a radio resource of the first base station for the mobile subscriber station according to the message, and after releasing the radio resource, transmitting data (S42) to be transmitted to the mobile subscriber station to the first base station to the second base station.