PURPOSE: A method and an apparatus for checking the transmission success or failure of a medium access control protocol data unit in a mobile communication system are provided to prevent a terminal from being operated unnecessary by judging the success or failure of an MAC PDU(Medium Access Control Protocol Data Unit). CONSTITUTION: A method for checking the transmission success or failure of a medium access control protocol data unit in a mobile communication system comprises the steps of: inspecting whether or not transmission resources for initial transmission are allocated in the retransmission point of an MAC PDU(505-520); and judging the success or failure of transmission by inspecting whether the feedback of an HARQ(Hybrid Automatic Retransmission reQuest) feedback is an ACK response or NACK response(525).