We model the relationship between a WLAN access point (AP) and a paying mobile station (MS) as a waiting-line auction [1] in which clients bids for transmission using the length of Contention Window (CW) as bid signals. The shorter CW is, the higher the access probability is, thus the higher pay to AP. The clients adapt their CWs based on the information acquired from current service load and wireless environment. Clients could achieve equilibrium bids according to Nash Equilibrium strategy.