A method and apparatus for scheduling data transmission in a mobile communication system supporting a packet data service without increasing downlink signaling overhead uses a receiver adapted to receive an absolute grant indicating an absolute value of an allowed maximum grant for data transmission. From a node B a relative grant indicating a change in the allowed maximum grant for data transmission is received. A controller determines whether the AG is received with a pre-assigned first identifier of a user equipment and the allowed maximum grant according to the absolute grant is updated and also the allowed maximum grant according to the relative grant is updated at the future transmission time interval when the absolute grant is received with the first identifier.