Abstract: 200nm AlN film is grown by pulse MOCVD.It’s Full width at half Maximum（FWHM） of XRD rocking curve is only 130arcsec and root mean square（RMS） roughness is 2.021nm.Used AlN as template,AlGaN films with different Al fraction are grown.High resolution XRD measurement indicate that compressive strain which AlN template apply to AlGaN films and tensile strain which come from coalescence of islands in AlGaN films increase with increasing Al fraction.When the Al fraction is about 0.67,the tensile strain and the compressive strain is in the balance,the AlGaN film has best quality.