This article reports the use of Gallium phosphide (GaP) and Ag/GaP nanoparticles, which can harness visible light to decompose organic dye in aqueous solution. The Ag(1.139 wt%)/GaP and Ag(5.225 wt%)/GaP nanoparticles were prepared by the liquid phase reduction of silver nitrate with hydrazine hydrate. The application of X-ray fluorescence and high-resolution transmission electron micrograph morphology has provided direct evidence of the presence of silver on the GaP nanoparticles. Under visible light, the experiments on the photocatalytic degradation of crystal violet in solution over the GaP and Ag/GaP nanoparticles were carried out. The results reveal that small size and number density of Ag domains deposited on GaP nanoparticles have enhanced photocatalytic efficiencies, as compared to large size and number density of Ag domains. This study suggests the potential of both GaP and Ag/GaP nanoparticles as photofunctional materials for waste-water cleaning.