As plug-in hybrid electric vehicles（PHEVs） are expected to be widely used in the near future， a mathematical model is developed based on the traditional security constrained unit commitment（SCUC） formulation to address the power system dispatching problem with PHEVs taken into account. With the premise of power system secure operation， both the economic benefits for PHEV users and the carbon-emission costs are taken into account. Then， the features of PHEVs as mobile energy storage units are applied to decouple the developed model into two sub-models， involving the unit commitment model and the charging and discharging scheduling model that includes AC power flow constraints. The optimal plug-in capacities for PHEVs and the schemes， including when and where charging and discharging occur， are obtained through a mixed integer programming algorithm and the Newton-Raphson load flow algorithm in addition to the optimal day-ahead unit commitment scheme. Finally， the feasibility and efficiency of the proposed model are verified with a 6-bus test system and the IEEE 118-bus test system.