Organic　Rankine　Cycle　(ORC)　is　a　promising　technology　for　converting　low　or　medium　temperature　heat　into　power.　In　order　to　improve　the　thermodynamic　efficiency　of　ORC　systems,　adding　a　regenerator　to　the　system　has　been　widely　discussed.　However,　this　needs　additional　pipes　and　devices　and　they　will　produce　additional　pressure　drop.　Considering　both　the　contributions　of　the　regenerator　and　the　power　losses　caused　by　additional　pressure　drop,　a　theoretical　criterion　for　evaluating　the　thermodynamic　effectiveness　of　adding　a　regenerator　was　proposed　by　means　of　mathematic　and　thermodynamic　equations.　The　core　of　the　criterion　is　a　new　concept　named　Critical　Pressure　Drop　produced　by　the　Regenerator　(CPDR).　A　negative　value　of　CPDR　indicates　that　adding　a　regenerator　will　not　contribute　to　the　thermodynamic　efficiency　of　ORC　systems　in　any　case.　An　effective　regenerator　needs　a　positive　value　of　CPDR,　and　requires　that　the　value　of　additional　pressure　drop　produced　by　introducing　the　regenerator　should　be　smaller　than　CPDR.　The　influences　of　evaporation　temperature,　condensation　temperature,　expander　efficiency,　regenerator　effectiveness　and　working　fluids　on　CPDR　were　also　examined.　It　was　found　that　lower　evaporation　temperature　or　using　working　fluids　with　higher　critical　temperature　tends　to　reduce　the　value　of　CPDR.