We investigated substrate orientation and As species dependence of GaAsxP1-x layers grown on (100), (n11)A, and (n11)B GaAs substrates (n=3, 4, and 5) by molecular beam epitaxy at a substrate temperature of 535degreesC. When GaAsP layers were grown using As-4 and P-2 molecular beams, the As content of the layers were almost independent of the substrate orientation and were 0.92-0.94. When As-4 and P-2 molecular beams were supplied, the As contents of (100) and (n11)B GaAsP layers were almost constant (xsimilar to0.88), while (n11)A GaAsP layers had smaller As contents. The (411)A GaAsP layer had the smallest As content of 0.75. Based on the process in which As-4 molecules dissociate into 2As(2) or vaporize on the surface, the results indicate that dissociation and vaporization of As-4 is Strongly dependent on the substrate orientation, while the incorporation process of As-2 is almost independent of the substrate orientation. The results also strongly suggest that the B-step edges provide the effective sites for the dissociation and the adsorption of As-4 molecules.