A method for performing buffer status reporting (BSR) in a user equipment of a wireless communication system comprises forming a MAC PDU comprising a padding field with a size smaller than a first predefined value, selecting a first logic channel group from a plurality of logic channel groups of the user equipment according to priorities and data amounts of uplink buffers of the plurality of logic channel groups, carrying information about a data amount of an uplink buffer of the first logic channel group with a short-format BSR control element, and carrying the short-format BSR control element with the padding field of the MAC PDU for a network of the wireless communication system.