A Multi-User-Multiple Input Multiple Output (MU-MIMO) transmission method performed by an AP in a Wireless Local Area Network (WLAN) system is provided. The method includes transmitting an MU-MIMO initiation message to a destination STA which is a target of a MU-MIMO transmission, the MU-MIMO initiation message informing that MU-MIMO transmission will be initiated, receiving a sounding frame transmitted by the destination STA as a response to the MU-MIMO initiation message and performing MU-MIMO transmission on data by beamforming based on channel information obtained from the sounding frame. The sounding frame includes precoded and virtualized channel information Hvirtual between the AP and the STA. A dimension of the Hvirtual is lower than a dimension of channel information H between the AP and the STA.