This paper proposes DPS-MAC (Divided Preamble Sampling-MAC), a carrier sense Medium Access Control (MAC) protocol with ultra low power operations for wireless sensor networks. Due to battery-operated computing and sensing devices in wireless sensor networks, the development of MAC protocols that efficiently reduce power consumption is an important issue. DPS-MAC is a novel energy efficient protocol based on sampling preamble segments that have preamble offsets and next hop addresses. By using these parameters, DPS-MAC can overcome the drawbacks of long fixed preambles and overhearing in B-MAC. The divided preamble sampling technique provides not only very low power consumption when the channel is idle, but also a chance to avoid over-hearing without additional overheads compared to B-MAC. This technique makes it possible to extend the lifetime of the wireless sensor networks that contain a large number of nodes. The experiment results show that DPS-MAC protocol reduces much energy consumed by receiving compared to B-MAC and can support various features of wireless sensor networks efficiently.