The　organic　Rankine　cycle　(ORC)　is　a　popular　technique　used　in　the　utilization　of　low-grade　thermal　energy.　Among　wet,　dry,　and　isentropic　organic　working　fluids,　the　latter　two　types　are　more　appropriate　for　ORC　systems.　In　this　paper,　the　definition　of　turning　point　on　saturated　vapor　curve　of　dry　fluid　and　isentropic　fluid　was　given　according　to　the　shape　of　the　saturated　curve　of　working　fluids　in　a　T-s　diagram.　On　this　basis,　the　model　of　near-critical　region　triangle　was　established.　Using　this　model,　the　thermodynamic　performance　of　57　kinds　of　dry　and　isentropic　organic　working　fluids　in　ORC　was　evaluated.　The　performance　includes　the　relation　between　turning　point　temperature　and　cycle　thermal　efficiency,　the　relation　between　near-critical　region　triangle　area　and　cycle　thermal　efficiency,　the　relation　between　near-critical　region　triangle　area　and　exergy　at　turning　point　temperature,　the　relation　between　near-critical　region　triangle　area　and　reciprocal　value　of　slope　of　saturated　vapor　curve.　Moreover,　working　fluid　selection　was　also　conducted　in　terms　of　heat　source　type.　It　was　found　through　theoretical　analysis　results　that　the　popular　R123　is　an　acceptable　choice　especially　for　the　utilization　of　closed　type　heat　source.　Considering　it　will　be　phased　out　in　near　future,　then　cis-butene,　butane,　trans-butene,　and　isobutene　are　worth　studying　as　its　successor.　Dodecane　is　worthy　of　attention　and　further　research　and　it　can　be　a　good　choice　for　utilization　of　open　type　heat　source.