A series of AlInGaN films with different contents of Al and In were grown by metal-organic chemical vapor deposition on sapphire with GaN (>2μm) intermediate layer. Rutherford backscattering/channeling was used to measure the compositions and the crystal quality of the AlInGaN films. Combining with the high resolution x-ray diffraction, the lattice constants and the strain in perpendicular andthe parallel directions of the AlInGaN epilayer can be determined accurately bytheθ-2θ scan of (0002) and (1015) reflections and the reciprocal-space x-ray mapping. We find that different contents of In and Al can effect the strain in the quaternary AlInGaN films obviously, and with the help of Vegard's law we give an explanation for this phenomenon.