Natural　gas　is　considered　as　a　green　fuel　due　to　its　low　environmental　impact.　LNG　contains　a　large　amount　of　cold　exergy　and　must　be　regasified　before　further　utilization.　ORC　(Organic　Rankine　Cycle)　has　been　proven　to　be　a　promising　solution　for　both　low　grade　heat　utilization　and　LNG　cold　exergy　recovery.　Due　to　the　great　temperature　difference　between　the　heat　source　and　LNG;　the　efficiency　of　one-stage　ORC　is　relatively　small.　Hence,　some　researchers　move　forward　to　a　two-stage　Rankine　cycle.　Working　fluid　plays　a　quite　important　role　in　the　cycle　performance.　Working　fluid　selection　of　a　two-stage　ORC　is　much　more　challenging　than　that　of　a　single-stage　ORC.　In　this　paper,　a　two-stage　ORC　is　studied.　Heat　source　temperatures　of　100,150　and　200　degrees　C　are　investigated.　20　substances　are　selected　as　potential　candidates　for　both　the　high　and　low　Rankine　cycles.　The　evaporating,　condensing　and　turbine　inlet　temperatures　of　both　Rankine　cycles　are　optimized　by　PSO　(Particle　Swarm　Optimization).　The　results　show　that　the　best　combination　for　heat　source　temperature　of　100　degrees　C　is　R161/R218　with　the　maximum　exergy　efficiency　of　35.27%.　The　best　combination　for　150　degrees　C　is　R161/RC318　with　the　maximum　efficiency　of　37.84%　and　ammonia/ammonia　with　the　maximum　efficiency　of　39.15%　for　200　degrees　C.　Fluids　with　intermediate　critical　temperature,　lower　triple　point　temperature　and　lower　normal　boiling　temperature　are　good　candidates.