In　this　study,　a　dual　loop　ORC　(organic　Rankine　cycle)　system　is　adopted　to　recover　exhaust　energy,　waste　heat　from　the　coolant　system,　and　intercooler　heat　rejection　of　a　six-cylinder　CNG　(compressed　natural　gas)　engine.　The　thermodynamic,　heat　transfer,　and　optimization　models　for　the　dual　loop　ORC　system　are　established.　On　the　basis　of　the　waste　heat　characteristics　of　the　CNG　engine　over　the　whole　operating　range,　a　GA　(genetic　algorithm)　is　used　to　solve　the　Pareto　solution　for　the　thermodynamic　and　heat　transfer　performances　to　maximize　net　power　output　and　minimize　heat　transfer　area.　Combined　with　optimization　results,　the　optimal　parameter　regions　of　the　dual　loop　ORC　system　are　determined　under　various　operating　conditions.　Then,　the　variation　in　the　heat　transfer　area　with　the　operating　conditions　of　the　CNG　engine　is　analyzed.　The　results　show　that　the　optimal　evaporation　pressure　and　superheat　degree　of　the　HT　(high　temperature)　cycle　are　mainly　influenced　by　the　operating　conditions　of　the　CNG　engine.　The　optimal　evaporation　pressure　and　superheat　degree　of　the　HT　cycle　over　the　whole　operating　range　are　within　2.5-2.9　MPa　and　0.43-12.35　K,　respectively.　The　optimal　condensation　temperature　of　the　HT　cycle,　evaporation　and　condensation　temperatures　of　the　LT　(low　temperature)　cycle,　and　exhaust　temperature　at　the　outlet　of　evaporator　1　are　kept　nearly　constant　under　various　operating　conditions　of　the　CNG　engine.　The　thermal　efficiency　of　the　dual　loop　ORC　system　is　within　the　range　of　8.79%-10.17%.　The　dual　loop　ORC　system　achieves　the　maximum　net　power　output　of　23.62　kW　under　the　engine　rated　condition.　In　addition,　the　operating　conditions　of　the　CNG　engine　and　the　operating　parameters　of　the　dual　loop　ORC　system　significantly　influence　the　heat　transfer　areas　for　each　heat　exchanger.　(C)　2016　Elsevier　Ltd.　All　rights　reserved.