Research activity in the area of medium-access control protocols of wireless sensor networks (WSN) has grown dramatically in the past few years. A number of MAC protocols are proposed to achieve energy efficiency upon the limitation of WSN which are usually deployed in a special environment, assigned with long-term work, and supported by limited battery. It is found that there is a trade-off between energy efficiency and transmission delay, thus duty cycle of fixed sleep/listening MAC protocols has to be adjusted carefully to achieve the best performance. Taking the challenge to design an adapted listening mechanism, A-MAC is designed to adaptively and dynamically adjust the duty listening time based on traffic load. With A-MAC, it can achieve more power efficiency in low traffic load and much less transmission latency comparing to existing MAC protocols. We simulate A-MAC, and the simulation result shows that A-MAC can significantly prolong the lifetime of network when traffic is low and reduce packet delivery latency.