Disclosed is a method for transmitting a buffer status report (BSR) by a terminal performing a handover procedure from a first network element to a second network element in a mobile communication system, including identifying a data packet available for transmitting and packet data convergence protocol (PDCP) control information, generating a BSR based on the data packet and the PDCP control information, the PDCP control information including header compression control information and a PDCP status report indicating which PDCP data is missing, and transmitting the generated BSR to the second network element, wherein the PDCP status report is transmitted to the second network element based on the BSR as a first PDCP PDU during the handover procedure performed by the terminal.