A method and device for transmitting a plurality of data units in a wireless local area network (WLAN) are discussed. The method includes obtaining by a station (STA) a transmission opportunity (TXOP), wherein the STA has a right to initiate frame exchange sequences during the TXOP; transmitting during the TXOP, a first data unit of the plurality of data units, wherein the first data unit comprises a signal field including information related to a bandwidth of the first data unit; selecting a bandwidth of a second data unit of the plurality of data units to be narrower than the bandwidth of the first data unit; and transmitting during the TXOP, the second data unit, wherein the second data unit comprises a signal field including information related to the bandwidth of the second data unit, wherein the first data unit is transmitted immediately before the second data unit.