Adsorption of CH3O at four sites (top, bridge, hcp, fcc) on Au(111) surface has been investigated by density functional theory method at the generalized gradient approximation level. We have performed calculations on adsorption energies, structures, Mulliken charges and vibrational frequencies of CH3O on Au(111) surface with full-geometry optimization. The predicted results are compared with the available experimental observation. The calculated CH3O adsorption structure and stretching vibrational frequencies agree well with experimental ones, and precise determinations of adsorption sites are carried out. The most favorite adsorption on Au(111) occurs at the bridge site, and O-C axis is tilted to the surface. However, on hollow sites (hcp, fcc) the species is adsorbed in an upright geometry (pseudo-C3v local symmetry).