Cu2ZnSnS4(CZTS) nanoparticles were prepared by the ball milling method, and CZTS layers were annealed under the Se vapor (selenization) to form CU2ZnSn(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.