The electric potentials for substorms are calculated from the three-dimensional MHD simulation for solar wind-magnetosphere coupling developed by Ogino <1986>, combined with the average conductivity maps in the ionosphere used in the work of Kamide et al. <1996>. For this combination, an electrostatic model of the ionospheric electric potentials is utilized, i.e., j∥ = ∇ ¿ (Σ ¿ ∇Φ), where j∥ is field-aligned current, Σ is ionospheric conductivity tensor, and Φ is ionospheric electric potential. The ionospheric potentials for the different phases of substorms are calculated through mapping field-aligned currents from the global MHD simulation onto the polar ionosphere. Comparing the present results with Kamide et al. <1996>, it is found that the realistic potential patterns for the growth and expansion phases of substorms can be obtained by the present electrostatic model. Owing to the lack of the magnetosphere-ionosphere coupling mechanism in the present study, however, the simulated ionospheric potential appears to be unrealistic for the peak of substorms. Although substorms are generated by north-to-south turnings of IMF in this model, the electric potentials for substorms are found to be affected only by southward IMF. It is also shown that the cross-polar cap potential drops are highly correlated with the interplanetary electric field for the growth and expansion phases of substorms. While the potential drops are also found to correlate well with the solar wind density, this positive correlation is opposite to what theoretical studies indicate. This is because the intensity of the field-aligned currents originating from the magnetopause, which is proportional to the electric potential, increases with the solar wind density in the simulation.