The present invention is a method for transmitting a data packet by a user terminal in a mobile communication system. The method enters a power saving mode for transmitting the data packet only if a channel state satisfies a transmission threshold value, compares the index indicating the channel state and the transmission threshold value upon occurrence of a data packet to be transmitted, starts transmission of the data packet if the index indicating the channel state is greater than the transmission threshold value, and proceeds with the transmission of the data packet until the time set by the timer that starts at the same time as the transmission of the data packet expires.