This　paper　addresses　the　issues　related　to　the　design　of　optimum　structure　for　lightweight,　flexible　manipulators　under　the　total　weight　constraint.　The　optimum　design　problem　is　formulated　by　establishing　a　mathematical　model　of　flexible　manipulator　based　on　the　finite　difference　method,　the　optimal　solution　is　obtained　by　using　an　improved　real-code　genetic　algorithms　(GA).　Numeric　simulation　has　been　conducted　and　the　results　have　demonstrated　the　effectiveness　of　the　proposed　method.