Strain in InGaAs and AlAs epilayers grown on GaAs by atmospheric pressure metal organic chemical vapor deposition has been analysed using X-ray double crystal diffraction. It is seen that the epilayers grow tilted with respect to the substrate lattice. The measured tilt angles of the lattice matched epilayers agree well with Nagai`s model But the magnitude and the direction of the tilt of the partially relaxed epilayers were rather random and thought to be dependent on the details of the growth condition. In the partially relaxed epilayer, it is observed that the lattice relaxation is greater in [11 ̄0] than in [110]. So that, the epilayer lattice is thought to assume the orthorhombic distortion. The measured degree of the relaxation as a function of epilayer thickness agrees better with the half loop nucleation model than the mechanical equilibrium model or the energy balance model which have been used widely.