A method and a device for transmitting a frame in a wireless LAN are disclosed. A method for transmitting a data unit in a wireless LAN can comprise the steps of: transmitting, by an AP, an RTS frame for medium protection to a plurality of STAs through a plurality of channels; receiving, by the AP, a CTS frame in response to the RTS frame from each of the plurality of STAs through a first allocation channel, wherein the first allocation channel is at least one of the plurality of channels, which is determined on the basis of the RTS frame; and transmitting, by the AP, downlink data to each of the plurality of STAs through a second allocation channel on the basis of OFDMA in overlapped time resources, wherein the second allocation channel is at least one of the plurality of channels.