The present invention relates to a method of processing a data unit by a transmitter in a wireless communication system. In particular, the method includes the steps of: submitting, a lower layer, a first data unit with performing data compression algorithm to the first data unit; receiving, from a network, a control data unit for resetting the data compression algorithm; instructing the lower layer to discard the first data unit; resetting the data compression algorithm; and submitting a second data unit with performing the reset data compression algorithm to the second data unit.