The electric vehicles (EVs) can introduce new operation strategies, such as charging management and vehicle-to-grid control, which could provide a considerable level of distributed storage to the power grid. In this paper, a partial decomposition method which is based on the Lagrangian relaxation framework is proposed for the EV charging control in transmission-constrained power systems. The partial decomposition method helps reduce the number of dual multipliers and stabilize the iterative process. The proposed partial decomposition framework is applied to a day-ahead SCUC algorithm which can be easily implemented in the existing hierarchical power system operations. The proposed EV charging control method can not only help reduce the total generation cost of power systems but also alleviate the transmission grid congestion. The feasibility of the proposed method is validated by the case studies applied to the modified IEEE-RTS1979.