IEEE 802.15.4 Wireless Sensor Networks (WSNs) share the 2.4 GHz Industrial, Scientific, and Medical (ISM) license-free band with many other wireless technologies such as IEEE 802.11b/g Wireless Local Area Networks (WLANs). Because of the low-power, however, IEEE 802.15.4 WSNs are potentially vulnerable to the interference introduced by the other wireless technologies such as IEEE 802.11b/g WLANs, which have much higher power. Particularly, in the presence of heavy interference, IEEE 802.15.4 nodes can hardly get a chance to access the channel, which could result in discarding a large amount of packets. In this paper, we propose a decentralized approach to help IEEE 802.15.4 nodes mitigate interference. By adaptively and distributively adjusting Clear Channel Assessment (CCA) thresholds of IEEE 802.15.4 nodes in the presence of heavy interference, the approach can substantially reduce the amount of discarded packets due to channel access failures, and therefore significantly enhance the performance of IEEE 802.15.4 WSNs. The approach is robust, responsive and easy to be implemented at a low cost. The effectiveness of the approach is validated by OPNET simulation.@en