The energy barriers for single adatom diffusion of Si and Ge on the (100)-2x1 reconstructed surfaces of Si and Ge are calculated by Modified Embedded Atom Method (MEAM). The results for Si/Si are compared with other theoretical works. The activation energies for diffusion along and across the dimer row are 0.27 and 0.60 eV for Si/Si and 0.22 and 0.48 eV for Ge/Ge, respectively. Anisotropic diffusion is also suggested for Ge/Si. However, larger activation energy for Ge/Si than Si/Si and small diffusion anisotropy for Si/Ge are contradictory results to experiments, which suggests that the treatment of the Si-Ge interaction should be improved.