©　2018　American　Society　of　Civil　Engineers.　This　paper　proposed　a　gas　turbine　and　organic　Rankine　cycle　(GT-ORC)　combined　cycle　to　further　improve　the　energy　efficiency　of　gas　turbines.　Alkanes,　linear　siloxanes,　and　aromatics　were　selected　as　working　fluids　for　the　bottoming　organic　Rankine　cycle.　Based　on　the　mathematical　model　and　solution　procedure　proposed,　a　thermodynamic　comparison　of　GT-ORC　combined　cycle　was　conducted　with　pure　and　mixture　working　fluids.　Simulation　results　showed　that　the　mixtures　made　the　combined　cycle　achieve　higher　efficiency　than　pure　fluids.　Compared　with　conventional　steam　Rankine　cycles,　the　organic　Rankine　cycle　had　larger　potential　in　recovering　exhaust　heat　from　gas　turbines.　The　thermodynamic　analysis　showed　that　the　thermal　efficiency　of　the　bottoming　cycle　increased　with　the　rise　of　turbine　inlet　pressure.　Besides,　the　ORC　net　power　was　maximum　at　the　optimum　turbine　inlet　pressure.　Four　commercial　gas　turbines　with　different　exhaust　temperatures　(553-778　K)　were　also　examined,　and　results　indicated　that　the　Trent　60　combined　cycle　achieved　the　highest　thermal　efficiency　of　56.48%.　For　gas　turbine　of　different　power　levels,　the　toluene/benzene　mixture　was　more　suitable　in　recovering　waste　heat　from　small　and　medium　size　gas　turbines,　while　the　cyclopentane　was　more　applicable　for　microgas　turbines.