IEEE 802.11 Distributed Coordination Function (DCF) guarantees fair transmission opportunity to stations. Channel occupation of low rate stations in IEEE 802.11 Wireless Local Area Network (WLAN) causes the performance anomaly resulting in the degradation of overall network throughput. Using cooperative communications between a high rate station and a low rate station, performance anomaly can be mitigated in IEEE 802.11 WLAN. In this paper, we propose a new cooperative Medium Access Control (MAC) protocol to remedy the performance anomaly and to improve transmission opportunities for low rate stations. We show that the proposed cooperative MAC protocol can enhance overall performance (e.g., throughput and MAC delay).