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Online power system stability assessment during restoration has not been strongly addressed yet. The introduction of wide area measurement systems (WAMS), however, made it possible to monitor the stability online. This study technically presents a detailed analysis of stability during restoration using WAMS. The power system build-up strategy is used as the restoration approach, based on which the early and the last stages of restoration are to divide the power system into islands and interconnect them, respectively. In fact, the practical use of WAMS at the early stages of restoration provides precise determination of generators loading steps. Moreover, unification of phase angle references at different islands and presentation of tie line energising priority list are significant benefits of WAMS at the last stages of restoration. The New England 39-bus power system is used to demonstrate the proposed algorithm and verify the results. The proposed method is also applied on the IEEE 118-bus system as a large-scale power system to prove its capability in practical systems.