The　organic　Rankine　cycle　(ORC)　system　powered　by　exhaust　heat　has　great　potential　in　improving　engine　performance.　Many　optimizations　of　the　only　ORC　system　were　conducted,　while　the　existing　literature　pays　limited　attention　to　the　optimization　of　the　engine-ORC　combined　system.　By　considering　the　importance　of　interaction,　cooperation,　and　influence　between　the　engine　and　ORC　system,　a　global　optimization　of　the　diesel　engine-ORC　combined　system　(herein,　the　combined　system)　is　conducted　in　this　paper　with　respect　to　power　output　and　fuel　economy.　A　GT-Suite　model　of　the　combined　system　and　a　GT-Suite/Simulink　co-simulation　model　are　proposed　to　obtain　the　optimum　operating　parameters　of　the　engine　and　the　ORC　system　under　various　operating　conditions.　Furthermore,　the　effects　of　the　operating　parameters,　namely,　exhaust　valve　timing,　injection　timing,　expander　speed,　and　pump　speed,　are　evaluated　on　the　combined　system.　In　addition,　models　of　the　engine　and　the　ORC　system　are　calibrated,　and　a　particle　swarm　optimizer　(PSO)　is　designed　and　adopted　for　global　optimization.　Optimization　results　show　improvements　of　3.24%　and　3.13%　on　the　power　output　and　brake　specific　fuel　consumption　(BSFC),　respectively,　with　full　engine　load　when　the　engine　is　operated　at　3600　r/min.　In　the　optimization　of　fuel　economy　with　partial　engine　load,　a　maximum　reduction　of　5.71%　on　the　BSFC　of　the　combined　system　is　obtained　at　3600　r/min　engine　speed.