The adsorption states of ethylene on Si(100)c(4×2) were studied by means of high-resolution electron energy loss spectroscopy at low temperatures. We found that some of ethylene molecules are weakly bonded to the surface as a π-complex type at low temperatures (∼50 K). This species is converted to di-σ bonded ethylene by heating. Thus, we conclude that the π-complex species is a stable intrinsic precursor for the cycloaddition reaction (the di-σ bond formation) of ethylene on Si(100)c(4×2). The activation energy from the π-complex precursor to the di-σ chemisorption state is experimentally estimated to be 0.20 eV.