By means of density functional theory on the basis of generalized gradient approximation, the equilibrium geometries and electronic properties of different hydrogenated diamond (100) surfaces were calculated. The results indicate that in the case of 2×1 reconstruction structure, the relaxation occurs along directions parallel and vertical to the surface. However, the relaxation does not appear in the direction parallel to the surface in the case of 1×1 structure. In addition, for the clean surfaces with C(100)-2×1, C(100)-2×1 ∶0.5H and C(100)-1×1 ∶1.5H structures, empty surface states exist in their band gaps. But with regard to the C(100)-1×1 ∶2H and C(100)-2×1 ∶H surface structures, which are configurations with full hydrogen coverage, the empty surface states shift upwards into the conduction band. With the help of analysis of charge density distribution, the inducing mechanisms of surface states were investigated.