Self-diffusion of Ag/Ag (100) is investigated by molecular dynamics with the semiempirical surface-embedded-atom method (SEAM) potential. Many diffusion mechanisms, such as hopping, simple exchange, complicated exchange and a new mechanism named multiple-hopping are observed. Another model for explaining the complicated exchange mechanism is given. From Arrhenius- behavior of the adatom and the statistical results, we obtain the active energies for simple- exchange and hopping mechanisms respectively, they are 0.39 and 0.47eV.