Most of sensor MAC protocols have a fixed duty cycle, which performs poorly under the dynamic traffic condition observed in event-driven sensor applications such as surveillance, fire detection, and object-tracking system. This paper proposes a traffic-aware MAC protocol which dynamically adjusts the duty cycle adapting to the traffic load. Our adaptive scheme operates on a tree topology, and nodes wake up only for the time measured for the successful transmissions. By adjusting the duty cycle, it can prevent packet drops and save energy. The simulation results show that our scheme outperforms a fixed duty cycle scheme [5] and B-MAC [9], hence, it can achieve high packet fidelity and save energy.