A method for establishing a radio bearer in a User Equipment (UE) in a mobile communication system is disclosed. The method includes receiving configuration information about a specific protocol layer, the configuration information indicating separate configuring of a downlink data block format and an uplink data block format for an entity of the specific protocol layer, wherein the entity of the specific protocol layer has both a transmitting side and a receiving side, and configuring the downlink data block format and the uplink data block format separately for the entity of the specific protocol layer according to the received configuration information.