The penetration of a large number of plug-in hybrid electric vehicles (PHEVs) has introduced severe challenges to power grid operation. In this paper, the grid-connected PHEV model is first proposed to reflect the charge/discharge property of PHEV. Then, a simulation approach is developed to study the probability distributions of the aggregate load and available vehicle to grid (V2G). capacity of PHEVs. Simulation results show that the charging and discharging capacities have the characteristic of normal distribution.