The invention relates to a method and apparatus for reporting a buffer status to be, in particular, the terminal base station in a mobile communication system of the mobile communication system. How the UE reports the buffer status information to a base station in a mobile communication system according to the invention, for a single radio bearer group (RBG) for transmitting the data stored in the buffer, a plurality of mapping the plurality of the buffer size and the code point by using a mapping table, a first buffer size element (BSE) and the 2 BSE process and, with the first BSE with RBG identifier indicating the RBG wherein the 2 BSE constituting the indicating the buffer status of the buffer a housing configured by the scheduling information (SI), comprises the step of transmitting to the base station. LTE (Long Term Evolution), SI (Scheduling Information), RBG (Radio Bearer Group), BSE (Buffer Size Element)