Migration from a masked region and lateral vapor-phase diffusion are the mechanisms of growth-rate enhancement and compositional change for InGaAsP/InP selective metal-organic vapor-phase epitaxy (MOVPE). A novel threshold mask width where the lateral vapor-phase diffusion starts to occur is proposed. When the mask width is less than the threshold mask width, the major mechanism of selective MOVPE is the surface migration from the dielectric mask region, and the lateral vapor-phase diffusion is very small. On the other hand, when the mask width is larger than the threshold mask width, the major mechanism of selective MOVPE switches to lateral vapor-phase diffusion. We discuss the effective migration length on a dielectric mask and the mechanism of the narrow-stripe selective MOVPE for several growth conditions by considering the concept of the threshold mask width.