A high resolution measuring method to determine lattice mismatch or the composition for a single ultra thin Ga x In 1− x As ternary layer is discussed. The maximum errors of lattice mismatch and composition x depend strongly on the uncertainty of band-offset and are, respectively, ±0.10% and ±0.01 in the case of a two-monolayer GaInAs single quantum well for band-offset uncertain at a conduction band of ΔE c = 0.42 ± 0.02 eV. The accuracy of this method is also calibrated using an X-ray diffraction technique for multiple quantum well structures of two-monolayer thick wells grown by digital epitaxy. This method may also be applied to GaInAsP quaternary.