The　investigation　was　conducted　on　the　matching　criterion　between　working　fluids　and　cycles　under　variable　working　conditions　for　the　organic　Rankine　cycle　(ORC).　The　ant　colony　algorithm　was　applied　to　examine　the　performance　of　the　basic　ORC　(BORC)　and　the　ORC　with　an　internal　heat　exchanger　(IHORC)　under　the　different　operating　conditions　(i.e.,　working　fluid　and　heat　source　temperature).　On　the　basis　of　the　results　calculated,　a　matching　criterion　between　the　working　fluids　having　different　critical　temperatures　and　the　heat　source　temperatures　was　achieved.　The　results　show　that　the　IHORC　displays　higher　exergy　efficiency　than　the　BORC.　The　BORC　system　has　a　higher　thermal　efficiency　and　output　net　power　when　the　heat　source　temperature　reaches　the　upper　limit　of　the　region;　when　a　working　fluid　with　high　critical　temperature　is　adopted,　the　condenser　runs　at　negative　pressure,　reducing　the　stability　and　economy　of　the　system.　At　a　given　heat　source　temperature,　the　working　fluid　can　be　selected　according　to　its　optimum　temperature　region.