The present invention relates to a method of transmitting a buffer status report (BSR) by a wireless node in a wireless communication system. In particular, the method includes the steps of: receiving first logical channel group (LCG) configuration information including identities of LCGs from a network; generating the BSR including a LCG field; and transmitting the BSR to the network, wherein a length of the LCG field is configured according to a highest value of the identities of LCGs.