Cu 2 ZnSnS 4 (CZTS) nanoparticles were prepared by the ball milling method, and CZTS layers were annealed under the Se vapor (selenization) to form CU 2 ZnSn(S,Se) 4 (CZTSSe) thin films. It was confirmed that the desorption of elements from the layer was occurred during the selenization. The suppression of the by-product formation was attempted by Se + Sn vapor or Se + S vapor supply during the selenization. In case of Se + Sn vapor supply, it was confirmed that the annealing temperature and the positioning of materials were import parameters. In case of Se + S vapor supply, the suppression of the by-product formation was achieved but the ratio between Se and S of CZTSSe was also affected.