Behavior of the Coulomb energy difference for light nuclei is explained in terms of the different values of theaverage Coulomb interaction between two particles. Coulomb energy difference according to shell model of light mirrornuclei in the Coulomb and exchange integrals in the formula can be explained with exponential-type wavefunctions. Inthis study, using the one-center expansion of exponential-type wavcfunctions in terms of Slater-type orbitals with thesame center, we derived formula for Coulomb energy difference of light mirror mulei.