We present a first-principles density-functional study for ethylene adsorption on the Si(100)-(2x1) surface. Ethylene is di-a bonded to the surface Si dimers with the adsorption energy of 1.81 eV. The dimer-maintained adsorption structure is found to be more stable than the dimer-cleaved one by 0.91 eV. Our calculations on the post-exposure of the ethylene-saturated surface to atomic hydrogen demonstrate the formation of Si-H bonds, the cleavage of the Si dimer bonds, and the minor changes for the chemisorbed ethylene, which is in good agreement with the recent experimental observations. (C) 1997 American Institute of Physics.