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In this paper, we propose an energy efficient MAC protocol for wireless sensor networks. Since sensor nodes are equipped with a limited energy of batteries, reducing energy consumption is a critical issue for extending network lifetime. To resolve this issue, we adopt a listen-sleep cycle as in S-MAC, allowing sensor nodes to turn their transceiver off during a sleep period. In S-MAC, sensor nodes have a fixed duty cycle and a synchronized scheduling in a virtual cluster. Hence, in S-MAC, it is not easy to adapt to the variation of network environment. Moreover, due to the synchronized scheduling, transmission collisions will increase resulting in energy waste and low throughput. To cope with such inefficiencies in S-MAC, we propose probability sensor MAC (PS-MAC), in which each node determines 'listen' or 'sleep' pseudo-randomly based on its own pre-wakeup probability and pre-wakeup probabilities of its neighbor nodes in each time slot. This allows the listen-sleep schedule of each transmitter and receiver pair to be synchronized while that of rest of nodes can be asynchronous. Therefore, collisions can be reduced even under heavy traffic conditions resulting in reducing energy waste and achieving high throughput. In addition, since pre-wakeup probability of each node can be adjusted adapting to the change of the network environment, by dynamically adjusting pre-wakeup probabilities of sensor nodes, system throughput can be further improved.