High-degree ordered Ga0.5In0.5P epilayers have been successfully grown by metalorganic chemical vapor deposition with phosphine (PH3) modulation. In this growth process, the PH3 was periodically switched into the reactor while the group III alkyls were constantly flowed. It was found that the interval of the PH3 switching has a significant influence on the degree of ordering and the surface morphology. From the 300 K photoluminescence (PL) measurement, the peak energy of the high-degree ordered Ga0.5In0.5P can decrease to 1.77 eV. The measured differences in the band-gap energy and refractive index between the high-degree ordered and disordered samples are 190 meV and 8%, respectively. Superior surface morphology, PL intensity, and background impurity concentration can be obtained for the ordered Ga0.5In0.5P grown by PH3 modulation.