Abstract: Quantum electrodynamics tells us that the coulomb potential is produced by exchanging virtual photon between the electric charge, but the most direct proofs are deduced from the coulomb potential in terms of the theory of virtual photon. The Klein-Gordon equation （□^2-m^2） Ф（x, t） = 0） is used to describe the scalar field with the mass m, zero spin and zero charge. The virtual photons can be described by the scalar field with its mass zero, so it satisfies the Klein-Gordon equation： □^2Ф（x, t） = 0. The coulomb potential is also deduced by using the Klein.-Gordon equation.

Key words: Klein-Gordon equation, Coulomb potential, virtual photon