Radial nanowire semiconductor devices have become a practical possibility in recent years. Hence, high-efficiency solar cells based on radial nanowire heterojunctions can now be realized. In this study, an electrostatic model for radial nanowire heterojunctions was developed by solving Poisson´s equation in cylindrical coordinates. In contrast to planar one-dimensional homo and heterojunctions, in this case, transcendental equations must be solved numerically to determine the radial space-charge dimensions. The electric field and potential distribution in the radial heterojunction are also obtained and graphed for different polarization voltages. As examples, InP/Si heterojunction and InP homojunction nanowire structures are electrostatically studied. This work is intended to be the basis for developing a more complete model for radial nanowire heterojunction solar cells under sunlight.