We employed density functional theory within the generalized gradient approximation to investigate the diamond (100) surface, with hydrogen and ethylene terminations. The radical chain reaction is investigated by slab models, and two possible adsorption ways are found according to our calculations. In addition, the electron affinity of H-terminated diamond is also calculated, which indicates that the existence of negative electron affinity of H-C (100) surface provides a necessary condition for initiating radical chain reaction. Our results also imply that ethylene molecules can form strong C-C covalent bonds with diamond surface, which make it more resistant against degradation processes. Furthermore, according to the analysis of electronic structures, we have found localized gap state above the valence band, which is mainly contributed by the interaction between diamond surface and ethylene molecule and can weaken the surface conductivity of the adsorbed diamond.