Abstract: Tremendous advances in communication, embedded computation and sensor technology have occurred in the development of wireless sensor networks, which are made by sensors, microprocessor and wireless communication interface, and have integrated collecting, processing and communication all-in-one. Time synchronization is a fundamental service for both design and application of wireless sensor networks. However, the complexity, energy, cost and size conflicts between traditional algorithms, such as NTP and GPS, and WSN prohibit their use in WSN. Because of the cost, power and size constraints of individual nodes, there are many new challenges. Developing time synchronization methods suitable for wireless sensor networks have attracted wide attention. Recently, many new algorithms are presented. But most of them sacrifice a lot of messages or complex computing for a good precision, so that much energy is consumed and life time of the whole network is shortened. In order to reduce messages and save energy, a new simple and low cost time synchronization algorithm for wireless sensor networks is proposed. It combines unidirectional reference broadcast and pair-wise mechanism, so that only one node in broadcast domain needs time synchronization, and the messages for time synchronization are reduced greatly. Finally, the performance of this algorithm is simulated.