The structural and electronic properties of 0.5 ML S adsorption on the Ga- and P-terminated GaP(001) (1 x 2) surfaces were studied by first-principles total-energy calculations. Sulfur adsorbates prefer to occupy the bridge sites on the Ga-terminated surface, consistent with the experimental results. Electronic analysis shows that the surface state around the Fermi energy (E-f) vanishes for S-adsorbed P-terminated surface at the substitution site. In the case of S-adsorbed Ga-terminated surface at the bridge site, the surface state at E-f is tremendously lowered but not completely diminish, which is in good agreement with the experiments. The S-induced work-function changes on the Ga- and P-terminated surfaces are 1.44 and 0.45 eV, respectively, indicating that some charge is transferred from the substrate to the S adsorbate. The Ga-S stretching vibrational frequency is calculated to be 37.61 meV.