This paper provides a concept for optimal placement of Phasor Measurement Units (PMUs) in state estimation of a power system for normal operating condition, as well as for double contingency one. State estimation has first been turned into an optimization problem in which the objective function is the Mean Square Error (MSE) in estimation process. In this paper, state estimation is studied based on the Weighted Least Squares (WLS) method. For the normal condition, Modified Shuffled Frog Leaping Algorithm (MSFLA) is used to find the optimal placement of PMUs. By taking into account of uncertainty, a multi-objective optimization procedure is formulated. To achieve this, MSFLA algorithm based on Pareto optimum method has been proposed here. The suggested strategy is applied on the IEEE 30-bus test system. This is done in several case studies to examine the optimal PMUs placement.