Cu(In, Al)Se2 (CIAS) thin films have been obtained by rapid thermal processing selenization of magnetron sputtering Cu-In-Al precursor thin films. The influence of selenization temperatures on the structural and optical properties of CIAS thin films has been investigated. The result reveals that the crystal structure of CIAS thin films depends on selenization temperature and the band gap energy has a red shift with the increase of selenization temperature. It is noted that the optimum selenization temperature of the CIAS thin films is 540℃, and it has a pure chalcopyrite structure with a band gap energy of 1.34 eV, which corresponding to the band gap energy of the theoretical maximum efficiency of solar cell absorber layer materials.