Wireless sensor networks play a key role in monitoring remote or inhospitable physical environments. One of the most important constraints is the energy efficiency problem. Power conservation and power management must be taken into account at all levels of the sensor networks system hierarchy. DPM (Dynamic Power Management) technology has been widely used in sensor networks. In this paper, we propose a new energy-efficient DPM, which is a modified sleep state policy developed by Simunic and Chdrakasan in [1] and combined with Optimal Geographical Density Control (OGDC) [22] to keep a minimal number of sensor nodes in the active mode in wireless sensor networks. Implementing dynamic power management with considering the battery status, probability of event generation and OGDC will reduce the energy consumption and prolong the whole lifetime of the sensor networks.