State-of-charge of power battery pack is an important parameter for electrical vehicle. Aiming at SOC estimation for Li-ion battery pack of multi cells in series, a Vmin state space model of battery pack is proposed. The minimal cell load voltage of the battery pack(Vmin) is the model outpt variable, and SOC is the model state variable. With Extended Kalman Filter applied, Dynamic state estimation of SOC is realized. The experiment was made to simulate the behaviors of battery pack in the actual work conditon of electrical vehicle. The result demonstrates that accurate and real-time estimation of SOC can be obtained with this approach.