Abstract: The geometry property and work-function change of Sn adsorption on Ni(100) surface are investigated by using the density functional theory (DFT) method of first-principles. Total energy calculations show that the substitutional surface alloy phase in the outermost layer is preferable. The buckling amplitude of the Ni(100)c(2×2)-Sn surface alloy is 0.049 nm. This result is almost identical to that previously reported on the basis of the low-energy alkali-ion scattering and low-energy electron diffraction (LEED) study (0.044±0.005 nm), and agrees well with the theoretical result using the full potential linearized augmented plane wave (FLAPW) method(0.036 nm). Unfortunately, there is a big difference between our result and earlier observations by constant-energy-loss electron-energy-loss holography (0.09 nm) and theoretical value of 0.018 nm based on CASTEP program. The work-function change due to Sn adsorption is found to be 0.31 eV, indicating the charge transfer from the substrate to the adsorbate.