The High quality InGaAsP materials, which wavelength shift is more than 177.5 nm with a small mask width of 0-40 μm, have been obtained by use of NSAG-MOCVD in the low growth pressure of 130 mbar. The composition variation, strain and the enhancement ratios of the vapor phase concentrations for TMIn and TMGa on the epilayer surface at the center of the mask opening to that in the maskless region with the mask width have been deduced, respectively. It is discovered that the ratio variation with the mask width is dependent on the concenration of TMIn and TMGa in the vapour phase when the mask width is less than the threshold mask width, and independent on that when the mask width is more than the threshold mask width. Moreover, the phenomena of In rich with the mask width increase has been explained rationally.