A method for a WLAN system including transmitting, by a first station (STA), a trigger frame triggering uplink multi-user (UL MU) transmission from a second STA, in which the trigger frame includes preference information related to an access category (AC) type for an aggregation frame to be generated by the second STA and limit information related to a maximum number of traffic identifiers (TIDs) for the aggregation frame. In response to the trigger frame, the first STA may receive a UL frame configured based on the aggregation frame and a Quality of Service (QoS) null frame which has an acknowledgement (ACK) policy field being set to no ACK. When the QoS null frame is aggregated, the preference information and the limit information may be disregarded.