A method of transmitting a spatial stream for multi user (MU)-multiple input multiple output (MIMO) in a wireless local area network system and a transmitter for performing the method are provided. The method includes transmitting, to a receiver, a management frame including group information to assign or change a position of a plurality of spatial streams corresponding to each of a plurality of groups, and transmitting, to the receiver, a frame including at least one spatial stream, wherein the group information includes a plurality of group indicators and a plurality of spatial stream (SS) indicators, each of the plurality of group indicators indicating whether the receiver is a member of each of the plurality of groups, each of the plurality of SS indicators indicating a position of the plurality of spatial streams corresponding to each of the plurality of groups.