The performance of the IEEE 802.11 protocol based on the distributed coordination function (DCF) depends on the number of competing terminals. We propose game theory-based extended H∞ filter approach to the problem of estimating the number of competing terminals. The proposed approach has an advantageous feature that the noise information of the dynamic state system and the measurement equation is not required in its applications as opposed to the case of the Kalman filter. Simulation results show that H∞ filter outperforms the Kalman filter in estimating the number of competing terminals in both saturated and non-saturated conditions. In particular, the performance of the proposed approach is more satisfactory in non-saturated condition.