In order to reduce the residual thermal stress in GaAs layer on Si substrate, we have introduced a porous region and a thin (similar to 10 nm) Si layer over that in between GaAs and Si substrate. A 1-mum-thick undoped GaAs layers were grown by using chemical beam epitaxial technique at different temperatures. Because of the presence of porous region the morphology of the grown layers was slightly rough. Photolumineseence improvement and reduction of surface roughness have been achieved by chemical mechanical polishing (CMP). The ex situ non-contact optical interferometer observation shows that the rms roughness values of GaAs epilayer after CMP is 4.6 nm, whereas the as-grown is 9.5 nm, From the result of low-temperature photoluminescence, it was found that a significant reduction of the biaxial tensile stress has been achieved. The results prove that the growth of GaAs on Si substrate with intermediate porous region is a promising approach for obtaining GaAs epilayers with less biaxial tensile stress. (C) 2002 Elsevier Science B.V. All rights reserved.