In this paper, the distributed estimation in Wireless Sensor Network (WSN) using optimal task scheduling is looked upon. The main goal is to prolong network lifetime while the target parameter is estimated with desirable precision. The lifetime is defined as the number of rounds accomplished before network becomes nonfunctional. In order to prolong network lifetime, we determine the optimal number of active sensor nodes and the number of samples provided by each of them based on the degree of certainty (utilizing linear programming). Sensor observations from environment are quantized into messages and then directly forwarded to a fusion center where a final estimation is performed. Simulation results confirm that our optimal proposed algorithm has achieved significant efficiency when compared with other heuristic methods.