A method of STA-initiated uplink (UL) aggregation is proposed in a wireless communication system. Under the STA-initiated UL aggregation, an STA can gain access to the medium through contention and after winning the TXOP, it passes the TXOP ownership to its AP to allow it to trigger UL MU transmission. Thus, the AP has increased chance of utilizing the medium while maintains fairness to both legacy APs and STAs. In addition, once AP takes over ownership of the TXOP, if it detects idle secondary channels, it can enable UL aggregation over the idle secondary channels, thereby fully utilizing the entire system bandwidth.