X-ray diffraction is widely used to measure thr lattice parameters in the semiconductor heterostructures. For asymmetric reflection, both the glancing incident geometry and the glancing exit geometry satisfy the Bragg diffraction conditions. However, the rocking curves of these two diffraction geometries have different peak widths as well as different peak separations between the epilayer and the substrate. The direction of thickness broadening in the reciprocal lattice bring not parallel to the normal of the reflection plane is responsible for the asymmetrical broadening in the rocking curve. An exact mathematical procedure is given to determine the lattice parameters of the epilayer from the normal reflex and one geometry of asymmetrical reflex. This procedure is very useful for some annealed SiGeC samples, since only glancing incident geometry can be measured.