Organic　Rankine　cycle　(ORC)　is　an　effective　approach　for　low-grade　energy　utilization.　It　is　important　to　improve　the　efficiency　of　an　ORC　when　the　heat　source　temperature　varies.　In　this　study,　a　method　of　selecting　zeotropic　mixture　for　a　bottoming　ORC　with　changeable　heat　source　temperature　is　presented.　The　effects　of　the　operation　conditions　of　the　marine　engine　and　the　ambient　temperature　are　investigated.　First,　the　optimal　performances　of　the　ORC　using　40　pure　working　fluids　are　determined　and　compared.　Then,　two　zeotropic　mixtures　benzene/m-xylene　and　cyclopentane/toluene　are　selected　and　the　mechanism　of　temperature　match　with　the　heat　source　and　sink　is　explored.　Finally,　the　performance　improvement　with　benzene/m-xylene　using　the　composition　adjustment　method　is　evaluated.　The　results　indicate　that　the　suitable　pure　working　fluids　are　isopentane　and　R245ca　for　a　low　exhaust　temperature　while　toluene　and　m-xylene　are　the　best　when　the　exhaust　temperature　is　high.　Using　the　zeotropic　mixtures　benzene/m-xylene　and　cyclopentane/toluene　can　obtain　a　high　performance　over　the　operation　range　of　the　marine　engine.　When　the　exhaust　temperature　is　225　degrees　C,　the　net　power　and　exergy　efficiency　of　benzene/m-xylene　are　improved　by　6.9%-21.9%　and　6.5%-22.0%,　respectively,　compared　with　the　pure　fluids　benzene　and　m-xylene.　When　the　exhaust　temperature　increases　to　380　degrees　C,　these　improvements　decrease　to　1.9%-6.8%.　If　a　zeotropic　mixture　is　used,　the　critical　temperature　of　the　component　with　a　high-boiling　point　should　be　close　to　the　maximum　operation　temperature　of　the　heat　source,　and　the　temperature　glide　should　first　match　with　the　temperature　increase　of　the　heat　sink.　If　the　heat　sink　temperature　is　fixed,　it　is　impossible　to　enhance　the　ORC　performance　using　the　composition　adjustment　method.　However,　the　composition　adjustment　method　is　effective　when　the　temperature　of　the　heat　sink　varies　with　the　ambient　temperature.　The　net　power　and　exergy　efficiency　of　the　ORC　are　improved　by　up　to　21.9%　and　22%　in　winter　using　benzene/m-xylene.　However,　these　improvements　are　less　than　6.8%　in　summer.　(C)　2022　Elsevier　Ltd.　All　rights　reserved.