The　organic　Rankine　cycle　(ORC)　is　a　popular　technology　for　utilization　of　medium-low　grade　thermal　energy.　The　thermophysical　properties　of　working　fluids　can　greatly　affect　the　economic　performance　of　ORC　system　that　is　an　important　factor　affecting　its　application　and　development.　Heat　exchangers　account　for　the　largest　proportion　of　investment　in　the　whole　ORC　system.　In　this　paper,　a　curved　triangle　is　defined　to　describe　the　condensation　characteristics　of　dry　and　isentropic　working　fluids.　Using　this　model,　a　new　classification　of　dry　and　isentropic　working　fluids　and　their　application　characteristics　in　ORC　are　proposed.　Moreover,　a　method　used　to　determine　the　theoretical　optimal　or　the　theoretical　worst　condensation　temperature　of　dry　and　isentropic　working　fluids　is　proposed.　The　work　in　this　paper　may　offer　a　good　help　for　the　determination　of　practical　condensation　temperature　of　ORC　system　and　its　design　and　establishment.　(C)　2020　Elsevier　Ltd.　All　rights　reserved.