Previous investigations hypothesized different explanations for the coverage effect on vibration frequency shifts. The C-O stretching mode was most widely studied since its frequency was in the measurable range by IR spectra. The linearly increasing trend in stretching motion vibrational frequency has long been thought to be due to either intermolecular interaction or adsorbatemetal interaction . Instead, our result suggests that the increasing stretching frequency trend stems from the site conversion from a more hollow site to a more atop site with increasing coverage. On the other hand, for the horizontal motion, we propose that both site conversion and lateral repulsion induce the frequency change.