Abstarct: State estimation is widely considered as a tool to evaluate the real time power system prevailing conditions. State estimation algorithms could suffer divergence under stressed system conditions. This dissertation investigates impacts of variations of load levels and topology errors on the convergence property of the weighted least square (WLS) state estimator. To study the impact of the load increment on the convergence property of WLS state estimator, two types of load increment are utilized: one is the load increment of all load buses, and the other is a single load increment. In addition, phasor measurement unit (PMU) measurements are applied in state estimation. Hence, one can consider the divergence problem and improvement of state estimation accuracy. In this thesis, a new approach of the equality-constrained weighted least square state estimation with PMU measurements is proposed for improving the robustness of the state estimation. The real data and transmition network of khorasan are used as the test cases to in the estimation process. The simulation results show that adding PMU measurements could generally improve the robustness and acuracy of state estimation.