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In monitoring wireless sensor networks (WSNs) with event-driven applications, the generated traffic usually has bursty characteristics when an event occurs. Transient congestion in the networks would increase delay and packet loss rate severely, reducing the network performance greatly. Through the analysis on delay and packet loss rate in congestion, this paper proposes a congestion-aware mac protocol (C-MAC) for wireless sensor networks. In C-MAC, backoff delay of the nodes around contention area is adopted as a congestion indication. In normal state, the CSMA/CA is working well at sensor nodes. And when the congestion occurs, localized TDMA replaces the CSMA/CA in the congestion nodes. With the above mechanism, the congestion nodes only deliver their data during its assigned slots to control the contention overhead. Finally, we implement the C-MAC in our sensor network testbed. The experiment results show that the proposed MAC protocol could effectively operate on the congestion caused by burst information, and reduce both the delay and packet loss rate significantly.