Single-phased AgInSe 2 thin films were successfully prepared via depositing sol-gel derived precursors, followed by a selenization process. The pure-phased AgInSe 2 thin films were obtained at a selenization temperature as low as 400°C via adding the excess amount of In 3+ ions. Adjusting the In 3+ /Ag + molar ratios can effectively prevent the formation of impurities Ag 2 Se and AgIn 5 Se 8 in thin films. A Raman spectrum indicated that the obtained films belonged to chalcopyrite structure. The optical absorption revealed that the obtained AgInSe 2 had the band gap of 1.23 eV. According to the GIXD analysis of the prepared films, the formation mechanism of AgInSe 2 thin films is proposed. Se vapor reacts with Ag to produce Ag 2 Se at first, and then Se vapor subsequently reacts with In 2 O 3 and Ag 2 Se to form AgInSe 2 . The sol-gel assisted route with a selenization process was demonstrated to be a potential way for preparing AgInSe 2 thin films with densified microstructures.