We have investigated the C 1s photoelectron spectra of ethylene on the Si(100) (2×1) surface, using high resolution photoelectron spectroscopy. The vibrational structure has been observed in the C 1s spectra and the C-H stretching mode is found to be dominant. The vibrational splittings of C 1s state are very similar to those in the ground state. It is found that linear coupling model is applicable to this system and this excitation process is a Frank-Condon process. Curvature of the potential energy surface of C-H and C-D bond is almost unchanged upon core ionization.