Methylammonium　lead　iodide　(MAPbI(3))　is　one　of　the　most　widely　studied　perovskite　materials　for　solar　cells.　However,　the　poor　stability　of　MAPbI(3)　is　still　a　major　obstacle　for　commercial　applications.　In　this　work,　the　structural　stability,　electronic　and　optical　properties　of　MA(1-x)Cs(x)PbI(3)　have　been　systematically　investigated　by　first-principles　calculations.　The　results　showed　that　a　suitable　concentration　of　Cs　can　greatly　improve　the　phase　stability　of　MAPbI(3).　Moreover,　the　band　gap　of　MAPbI(3)　is　hardly　affected　by　doping　Cs.　These　compounds　keep　the　basic　features　of　MAPbI(3)　along　with　better　stability,　making　them　promising　candidates　for　highly　efficient　perovskite　solar　cells.　(C)　2019　The　Japan　Society　of　Applied　Physics