Wireless channel qualities vary and degrade due to fading and interference. Cooperative communication has characteristics of spatial diversity and virtual antenna, and thus it can improve communication efficiency significantly. One of the essential design issues in cooperative communication is partner selection mechanism. In this paper, we propose a partner selection algorithm which utilizes two decision factors: channel quality and unavailability. In this paper, channel quality is represented as signal-to-noise ratio, and unavailability of a node is the time duration it cannot act as a partner for its neighbors. When operating in cooperative mode, a source designates one neighbor, which satisfies the channel quality requirement and has the least unavailability value, as its partner. Furthermore, how to integrate the proposed partner selection mechanism with the IEEE 802.11 medium access control (MAC) protocol is elaborated. We evaluate our proposed algorithm by conducting simulations. From the simulation results, our proposed partner selection algorithm improves system throughput, reduces frame delivery time, and increases spatial channel reuse degree.