We examined experimentally how the surface morphology, photoluminescence (PL) and X-ray diffraction depend on the amount of compressive strain in the InGaAsP layers grown by metal-organic vapor-phase epitaxy (MOVPE). It was found that there are two critical values of strain in the relaxation process. One is the threshold strain value (epsilon(c1)) for the appearance of the cross-hatched pattern on the crystal surface and subsequent decrease in PL intensity. The other is that for the in-coherent growth (E-c2), which leads to the broadening of the full-width half-maximum (FWHM) in the Xray diffraction analysis. We also found that epsilon(c1) is in good accordance with the force-balance theory, while epsilon(c2) follows the energy-balance theory. We also examined their compositional wavelength (lambda(g)) dependence (lambda(g) of InGaAsP: 0.92 mum (InAsP), 1.15, 1.3 and 1.67 mum (InGaAs)). The epsilon(cl) of InAsP is approximately half those of the other materials. This is probably due to the relatively large Poisson's ratio for InAsP. On the other hand, epsilon(c2) of InGaAsP with of 1.15 and 1.3 mum is slightly smaller than that of InAsP or InGaAs. This can be explained by local strain fluctuation in a nanoscale region, i.e. the fluctuation is stronger for quaternary InGaAsP compared to that for ternary InAsP or InGaAs. These findings are very useful for the crystal growth of high-quality highly strained InGaAsP. (C) 2004 Elsevier B.V. All rights reserved.