Selenization is a key process to prepare Cu2ZnSnSxSe4-x(CZTSSe) thin films from sulfide for high efficient solar cell. To explore the effect of annealing atmosphere on the substitution of S by Se in Cu2ZnSnS4 (CZTS) thin films, different selenization atmosphere (Vacuum or Ar) are used. By examining the annealed sample, it is found that selenization under Ar atmosphere replaces S with Se more efficiently, resulting large grain and small band-gap. The reaction kinetics has been studied to explore such phenomena. It demonstrates that the substitution reaction is enhanced when the flowing Ar carries off S gas. Finally, the effect of annealing atmosphere on oxide-based CZTSSe devices has also been studied. The Vacuum selenized CZTSSe device shows a higher efficiency of 4.4%, due to the more appropriate band-gap. To the best of our knowledge, this is the best device performance in oxides nanoparticles-derived CZTSSe solar cell. (C) 2017 Elsevier Ltd. All rights reserved.