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Although the conventional duty cycle MAC protocols such as RMAC for wireless sensor networks (WSNs) perform well in aspects of saving energy and reducing end-to-end delivery latency, they are designed independently and require an extra routing protocol in the network layer to provide path information for the MAC layer. We propose a novel cross-layer duty cycle MAC protocol with data forwarding towards pipeline feature (P-MAC) for WSNs. P-MAC divides all sensor nodes in the network into different grades around the sink. Each node identifies its grade according to its logic hop distance to the sink and simultaneously establishes a sleep/wakeup schedule based on its grade. Those nodes in the same grade keep the same schedule, while which is staggered with the schedule of the nodes in the adjacent grade. Then a variation of RTS/CTS handshake mechanism is used to forward data continuously in a pipeline fashion from the higher grade nodes to the lower grade nodes and finally to the sink. No extra routing overhead is needed, and the superiority of duty-cycling is kept. The performance of PMAC and RMAC are evaluated and compared in terms of packet delivery latency and energy efficiency by OPNET simulation.