—This paper presents a new receiver-initiated sensor network MAC protocol, called CyMAC, which has the following unique features. It reduces the idle listening time of sensor nodes via establishing rendezvous times between neighbors, provides the desired relative delay bound guarantee for data delivery services via planning the rendezvous schedules carefully, and adjusts the sensor nodes’ duty cycles dynamically to the varying traffic condition. More importantly, CyMAC achieves the above goals without requiring time synchrony between sensor nodes. We have implemented and evaluated CyMAC in both TinyOS and the ns-2 simulator. Experimental and simulation results show that, comparing with RI-MAC – a state-of-the-art sensor network MAC protocol, CyMAC can always guarantee the desired delay bound for data delivery services and yields a lower duty cycle under reasonable delay requirements.