Because IEEE802.15.4-based wireless sensor networks (WSNs) and IEEE802.11-based wireless local area networks (WLANs) use same ISM frequency bands, they interferes each other. Based on the past researches, the interference makes severe impacts on the performances of WSNs rather than that of WLANs. This paper proposes a method to improve IEEE802.15.4-based WSNs performances coexisting with WLANs by transmitting only beacon frames in interference-free-channels. The method increases the reliability of beacon frames as well as overall channel utilizations. The effectiveness of the proposed method is evaluated through the extensive simulations.