Multi-user MIMO technology makes it possible for wireless nodes to successfully receive multiple packets from simultaneous transmitters in wireless networks. As it can provide more transmission opportunities without causing collisions, the network throughput performance can be dramatically improved. In this letter, we propose a medium access control (MAC) protocol, which allows more nodes to opportunistically transmit packets even though they do not exchange any control packets for transmission coordination if the AP can concurrently receive more packets due to the multi-packet reception capability. Through extensive simulations, we show that the proposed MAC protocol achieves significantly higher throughput performance in multi-user MIMO wireless networks.