A simple method for calculating the interaction force and energy per unit area between two dissimilar plates with high potentials at constant surface potential presented. Using Derjaguin's method and the improved Derjaguin' method, the expressions of the interaction free energy between two dissimilar spheres with high surface potentials are derived. These formulae may be divided into two groups: those for the strong interaction and those for the weak interaction. The juncture of strong and weak interactions is at kappah = 4 for dissimilar plates and at kappah = 2.8 for spheres The relative error is largest at this point, about 10%.