Surface migration of group V atoms (As and P) during molecular beam epitaxy was investigated for the first time by measuring lateral profiles of phosphorus content in GaAsP layers grown on (100) GaAs channeled substrates (CSs) with 12-16 mu m wide (n11)A side-slope facet region (n = 3, 4, and 5) using As-4 and P-2 beams. The P content x of the GaAs1-xPx layer on the (411)A side-slope region was 43% smaller (x = 0.11) than that on the (411)A flat substrates, while P contents of GaAsP layers on (311)A and (511)A side-slope regions were larger than those of GaAsP layers on a corresponding hat substrate. The observed lateral P content profile was well reproduced by simulation based on a conventional surface migration model. The migration lengths of As atoms, L-As, were determined as L-As((411)A) = 18 mu m, L-As((100)) = 28 mu m, L-As((511)A) = 33 mu m, and L-As((311)A) = 35 mu m, by comparing the observed and simulated profiles.