Decreasing　the　condensation　pressure　of　organic　Rankine　cycle　(ORC),　which　is　usually　limited　by　ambient　pressure,　can　greatly　increase　the　system　efficiency.　In　this　paper,　the　condensation　pressure　is　extended　from　traditional　ambient　condensation　to　vacuum　condensation.　Liquefied　natural　gas　(LNG)　is　used　as　the　cold　sink　to　generate　vacuum　condensation　pressures　between　0.001　MPa　and　0.1　MPa.　Nine　substances　with　no　or　low　flammability　are　chosen　as　the　working　fluids.　The　system　main　parameters　are　optimized　with　the　objective　of　maximum　exergy　efficiency.　The　results　show　that　the　exergy　efficiency　of　R227ea　increases　from　18.87%　to　26.03%　as　the　condensation　pressure　decreases　from　0.1　MPa　to　0.001　MPa.　For　some　substances,　however,　their　efficiencies　turn　to　decrease　as　the　condensation　pressure　is　approaching　0.001　MPa　and　their　optimal　condensation　pressures　are　different.　The　working　fluids　share　the　same　optimal　condensation　temperature　of　around　−90　°C　under　the　studied　conditions.　R227ea　is　a　generally　good　substance　because　of　its　higher　critical　temperature　to　match　with　heat　source　and　lower　triple　point　pressure　(temperature)　to　reach　lower　condensation　pressure　(temperature).　The　condensation　pressure　always　has　opposite　effect　on　exergy　destructions　of　condenser　and　steam　generator,　as　well　as　on　direct　expansion　turbine　and　ORC　turbine.　©　2022　Elsevier　Ltd