A method and an apparatus for reporting a buffer status of a buffer storing packet data to be transmitted by a user equipment for a scheduling assignment of an uplink packet data service in a mobile communication system supporting the uplink packet data service are disclosed. A user equipment stores packet data having a priority corresponding to a plurality of priority queues having inherent priorities and relating to at least one service, and transmits buffer status information containing queue identifiers of the priority queues and buffer payload information representing an amount of the packet data stored in the priority queues. Herein, the user equipment inserts the buffer status information into a header part of a protocol data unit for the uplink packet data service, inserts the packet data into a payload part of the protocol data unit, and then transmits the protocol data unit.