The uncoordinated charging load of large-scale electric vehicles (EVs) may increase the gap between peak load and valley load of future power grids. By designing proper charging pricing mechanisms for EVs to guide their charging behaviors, the negative effect caused by uncoordinated EV charging can be alleviated and the flexible EV charging load can even help achieve valley filling for the power grid and therefore increases the social welfare. This paper designs two valley-filling pricing mechanisms under non-cooperative and cooperative scenarios respectively that can intent EV owners to shift their charging schedules for flattening the power load profiles. In the non-cooperative scenario, each EV schedules its own charging power without cooperation with the other EVs, while in the cooperative scenario, all the EVs are controlled by an aggregator. Sufficient and necessary conditions of the valley-filling pricing mechanisms are derived for both the non-cooperative and cooperative scenarios. And the corresponding coordinated charging strategies are proposed for the two scenarios, respectively. Simulation results show that under the proposed pricing mechanisms, cost-minimizing charging schedules of self-interested EVs can also fill the load valley effectively. (C) 2016 Elsevier Ltd. All rights reserved.