A Multi-User-Multiple Input Multiple Output (MU-MIMO) transmission method performed by an Access Point (AP) in a Wireless Local Area Network (WLAN) system is provided. The method includes transmitting an MU-MIMO initiation message to a destination Station (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 between the AP and the STA. A dimension of the virtualized channel information is lower than a dimension of channel information between the AP and the STA.