The present invention relates to a method and apparatus for acknowledgement (ACK) transmission in a WLAN. A station receives a plurality of data frames from a plurality of other stations and then transmits an ACK for the plurality of data frames to the plurality of stations. The ACK is a multi-user (MU) block ACK frame which includes a plurality of block ACKs for the plurality of stations. One block ACK includes at least one ACK for at least one data frame that is received from one station.