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As a traffic adaptation technique in cluster-based wireless sensor networks (WSNs) employing IEEE 802.15.4 medium access control (MAC), the traffic adaptive 2-level active period control has been proposed in order to handle temporal and geographical nonuniform traffic fluctuations. This paper proposes a backoff control mechanism for the traffic adaptive 2-level active period control to enhance the system performance, especially transmission performance. The proposed mechanism autonomously adjusts the starting time of the backoff for the contention based channel accesses at the contention access period (CAP) specified by IEEE 802.15.4 MAC, and then, distributes the channel access timing to the wide range within the CAP, leading to lower channel access congestion. The results evaluated by computer simulation show that the proposed mechanism can improve the delay performance with keeping the throughput enhancement in the cluster-based WSNs under non-uniform traffic environments.