GaAs and GaAsP with the entire compositional range are grow on Si using an intermediate layer of GaAsP strained superlattices to relax the lattice mismatch. The orientation of the overgrown GaAs layer is found to be determined by the direction of the off-angle of the Si (100) surface. The grown layers are characterized by photoluminescence, x-ray diffraction, electro reflectance and DLTS. GaAs/GaAlAs double heterostructure lasers and GaAsP visible LED's are fabricated on Si substrates. The structural and electronic properties of the grown layers and the device performances are reported in this paper.