Proposed are a method and apparatus for transmitting a feedback frame in a wireless local area network (WLAN) system. Specifically, a first STA generates the feedback frame based on multi user (MU)-multi input multi output (MIMO) beamforming. The first STA transmits the feedback frame to a second STA. The feedback frame includes information related to a feedback subcarrier for a predetermined frequency band and a beamforming report field for the MU-MIMO beamforming. The feedback subcarrier includes a first subcarrier transmitted first in the feedback subcarrier and a second subcarrier of which a subcarrier index is determined based on a grouping value related to a subcarrier spacing. The beamforming report field includes a first signal to noise ratio (SNR) for the first subcarrier and a first differential SNR for the second subcarrier. The first differential SNR is an SNR difference between adjacent subcarriers included in the feedback subcarrier.