In this talk, we will give the scattering of the mass-critical nonlinear Klein-Gordon equations both in the defocusing and focusing case. We establish the linear profile decomposition, then by using the solution of the mass-critical nonlinear Schrodinger equation to approximate the large scale profile, we can prove the scattering result by the concentration-compactness/rigidity method developed by C. E. Kenig and F. Merle.