Comprehensive characterization of properties and first-principles calculations were first introduced to analyze tunable band gap Cu₂ZnSnS_4xSe_4(1−x) (CZTSSe) nanocrystals. Interesting aspects of composition and morphology were deeply explored. As the ratio of Se/(S + Se) rises, the parabolic nature of their tunable band gaps from 1.28 to 1.50 eV was revealed and verified.