A method of generating a buffer state of a packet data convergence protocol, hereinafter called PDCP, layer for a user equipment in a wireless communication system includes including volume information of packets, which have been submitted to a layer lower than the PDCP layer and have not been confirmed by the layer or by a PDCP status report, in a buffer state report of the PDCP layer, where the packets have not been confirmed by the layer due to buffer clearing of the layer.