Large　rupture　strain　(LRS)　fiber-reinforced　polymer　(FRP)　composites,　including　polyethylene　naphthalate　(PEN)　FRP　and　polyethylene　terephthalate　(PET)　FRP　composites,　have　become　increasing　popular　in　civil　construction.　PET　FRP　and　PEN　FRP　can　be　recyclable　and　thus　they　are　environmentally　friendly.　However,　the　durability　of　LRS　FRP　composites　under　extreme　environmental　conditions　(e.g.,　marine　environment　and　high　temperature)　has　not　been　understood.　To　this　end,　durability　tests　on　282　LRS　FRP　sheets　after　aging　in　alkaline/seawater　solution　with　different　temperatures　(25　&　DEG;C,　40　&　DEG;C,　60　&　DEG;C)　were　conducted　in　this　study.　The　microstructure　of　PEN　FRP　sheets　was　also　analyzed　using　scanning　electron　microscope　(SEM).　The　results　show　that　the　changes　of　the　elastic　modulus　of　the　first　linear　portion　and　the　elastic　modulus　of　the　second　linear　portion　of　PEN　FRP　sheets　after　aging　in　different　aging　temperatures　are　slight.　For　PEN　FRP　sheets　with　an　aging　period　of　365　days　in　60　C　solutions,　the　degradation　rate　of　the　tensile　strength　is　around　10%　for　both　alkaline　(pH　=　13.2)　and　seawater　solutions,　and　the　degradation　rate　of　the　tensile　rupture　strain　is　around　12.5%.　Additionally,　under　a　higher　exposure　temperature,　there　is　a　significant　decrease　in　the　tensile　strength　of　LRS　FRP　composites.　Furthermore,　it　shows　that　the　durability　of　PEN　FRP　sheets　was　significantly　affected　in　the　alkaline　environment　compared　to　that　in　the　seawater　environment,　and　the　alkaline　resistance　of　PEN　FRP　composites　is　better　than　that　of　PET　FRP　composites.