A numerical model of organic light emitting device (OLED) with metal/organic/metal structure was discussed on the basis of drift-diffusion equations in this paper. First，the influence of charges localized near the electrodes to the J-V curve was calculated and we got the same result of the literature. Second, the J-V curve of OLED with structure of ITO/PPV/Ca was simulated. In the simulation, the case of positive charge distribution localized near the anode was considered. The calculated J-V curve was in good agreement with the experiment. The charge creates an additional barrier and has a noted influence on the current density.