In this paper, we propose U-MAC, a medium access control protocol designed for wireless sensor networks. Nowadays, wireless sensor network are formed by a great quantity of sensor nodes, which are generally battery-powered and may not recharge easily. Consequently, how to prolong the lifetime of the nodes is an important issue while designing a MAC protocol. However, lowering the energy consumption may result in higher latency. Addressing on such tradeoff, U-MAC balances the tradeoff by utilization based tuning of duty cycle and selective sleeping after transmission. The experiment results show that our proposed U-MAC saves energy about 43% and reduce latency by 65% from S-MAC in a chain topology. In the cross topology, U-MAC also achieves 32% energy saving and 45% latency reduction from S-MAC