Electric vehicles (EVs) are not only controllable load, but also can serve as distributed energy storage unit. Therefore, ancillary services for the grid such as frequency regulation can potentially be provided by EVs with schedulable capacity. However, the schedulable capacity is limited by factors such as the constraints of user demand and the battery life. On the basis, a centralized control mode is applied to distributed EVs, and a model with charging and discharging static frequency characteristics of aggregators is developed considering EVs schedulable capacity. Then the load frequency control models including EVs for single area power system is developed for simulating load frequency control. The simulation results show that using EVs schedulable capacity for frequency regulation can not only reduce the system frequency deviation effectively, but also can decrease the reserve capacity of traditional frequency regulation unit, improving the electricity quality and economics of power system. Meanwhile, the result shows that user demand and battery life can not be neglected in EV participating in frequency regulation research.