Proposed are a method and apparatus for transmitting a feedback frame in a wireless LAN (WLAN) system. Specifically, a first station (STA) generates a feedback frame based on a multi input multi output (MIMO) beamforming procedure. The first STA transmits the feedback frame to a second STA. The feedback frame is transmitted through an aggregated channel in which a first channel and a second channel are aggregated. The feedback frame includes a first beamforming feedback matrix transmitted on the first channel and a second beamforming feedback matrix transmitted on the second channel. The feedback frame includes a third beamforming feedback matrix when the feedback frame is not transmitted through the aggregated channel. The number of columns of the third beamforming feedback matrix is set to Nc, and the number of rows of the third beamforming feedback matrix is set to Nr. Nc is determined based on the total number of spatial streams supported by the first STA. The number of columns of the first beamforming feedback matrix is set to Nc/2, and the number of rows of the first beamforming feedback matrix is set to Nr/2. The number of columns of the second beamforming feedback matrix is set to Nc/2, and the number of rows of the second beamforming feedback matrix is set to Nr/2.