The extensive integration of numerous plug-in electric vehicles（PEVs） into a power system could produce significant negative impacts on the secure and economic operation of the power system concerned， if the charging procedures of PEVs are uncoordinated. Given this， the hierarchical and zonal dispatching architecture is adopted and a new bi-level optimization model is presented for coordinating the charging/discharging schedules of the PEVs. The upper-level model is devoted to minimizing the system load variance so as to implement peak load shifting by dispatching each electric vehicle aggregator（EVA）， and the lower one is aimed at tracing the dispatching scheme determined by the upper decision-maker through figuring out an appropriate charging and discharging schedules throughout a specified day. Two highly efficient commercial solvers， AMPL/IPOPT and AMPL/CPLEX respectively， are employed to solve the developed optimization problem. Finally， a modified IEEE 30-bus system with 5 EVAs is employed to demonstrate the basic characteristics of developed model and method.