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Localization is one of the most important issues in Wireless Sensor Networks (WSNs). A lot of effort has been devoted to this issue which can be broadly classified into two categories: range-based schemes and range-free schemes. This paper proposes a range-free localization scheme by utilizing mobile anchors. Each mobile anchor is assumed to be able to know its current position with specific hardware such as GPS. It moves in the sensor field and periodically broadcasts its current position. In the proposed mechanism, we refer to the circle centered at an unknown node, with a radius equal to the maximum radio communication distance of the unknown node, as the sensing circle of the unknown node. Through the beacon messages broadcasted from the mobile anchors, several beacon points whose locations are approximately on this sensing circle of an unknown node can be derived. These points are called reference points of this sensing circle, and through them the estimated location of the center of the sensing circle can be easily computed via the MMSE (Minimum Mean Square Error) algorithm. Simulations of the proposed scheme are conducted with Matlab. Simulation results show that the proposed scheme outperforms other range-free mechanisms in terms of better location accuracy and less energy consumption for the mobile anchors.