Quaternary　compound　Cu2ZnSnX4　(X　=　S,　Se)　is　of　great　interest　to　many　researchers　for　photovoltaic　appli-cations　in　recent　years.　In　the　present　study,　the　first-principles　calculations　are　performed　to　investigate　the　influence　of　cation　substitution　on　the　different　physical　properties　of　Cu2ZnSnS4　(CZTS)　and　Cu2ZnSnSe4　(CZTSe).　The　difference　in　stability　between　two　Kesterite　and　Stannite　structures　is　examined　for　each　com-pound.　The　results　show　that　the　pristine　Kesterite　structure　will　be　transformed　into　the　Stannite　structure　when　the　Zn　atoms　are　replaced　by　the　Mg　or　Cd　atoms.　Our　results　are　consistent　well　with　the　available　literature.　The　mechanical　stability　of　six　compounds　are　proved　and　they　are　ductile　materials.　The　allowed　direct　elec-tronic　transition　is　found　for　all　compounds　at　the　Gamma　point.　The　band　gap　variation　is　analyzed　in　detail.　The　band　gaps　calculated　from　hybrid　functional　PBE0　of　the　studied　compounds　can　agree　well　with　the　experimental　data.　The　suitable　band　gap　values　of　1.12　and　1.28　eV　are　obtained　for　Cu2MgSnSe4　(CMTSe)　and　Cu2CdSnS4　(CCTS)　in　single-junction　solar　cells,　whereas　the　band　gaps　of　Cu2MgSnS4　(CMTS,　1.70　eV)　and　Cu2CdSnSe4　(CCTSe,　0.77　eV)　are　too　large　or　small.　The　band　gap　energy　difference　between　two　tetragonal　structures　is　shown　to　be　negligible　for　CMTS,　CMTSe,　CCTS,　and　CCTSe.　Furthermore,　the　optical　properties　are　compared,　and　CMTSe　and　CCTS　show　strong　light　absorption　capacity　from　300　to　800　nm.　Our　study　indicates　that　both　CMTSe　and　CCTS　are　the　promising　materials　for　solar　cell　applications.