Wireless sensor network (WSN) consists of more than one sensor deployed in a particular area.Typical usage of WSNs is to monitor environmental conditions, in such case, there are strict requirements of lifetime in monitoring applications, which demands the network to operate for at least a designated period of time The trade-off between sensor coverage and network lifetime has been studied extensively in the sensor network literature. One sensor is unable to cover the entire target environment. And due to the limited battery power, some targets will not be covered by any sensor eventually. The proportion of targets covered during the operation of WSN is a decreasing function of time. Thus, the aim of this paper is to find a scheduling scheme that extends the network operating time while maintaining a certain ratio of targets covered by some sensors. we propose a minimal energy-consuming schedule that accomplishes lifetime extension.