The　high-voltage　cascode　gallium　nitride　(GaN)　high-electron-mobility　transistor　(HEMT)　enables　high-frequency　and　high-efficiency　power　conversion.　The　parasitic　inductances　induced　by　traditional　packages　of　the　cascode　GaN　HEMT　device　significantly　deteriorate　the　device　switching　performance.　A　new　stack-die　packaging　structure　has　been　introduced　and　proved　to　be　efficient　in　reducing　package　related　turn-on　loss　and　turn-off　parasitic　ringing.　However,　the　thermal　dissipation　of　the　device　packaged　in　such　structure　becomes　the　limitation　for　further　pushing　the　operating　frequency　and　the　output　current　level　for　high-efficiency　power　conversion.　This　paper　focuses　on　the　analysis　of　thermal　performance　of　the　cascode　GaN　HEMT　device　in　different　packages　used　in　a　high-frequency　power　converter.　There　exist　several　possible　approaches　for　improving　thermal　performance　of　the　device.　Nevertheless,　it　is　not　convenient　and　cost-effective　to　put　every　method　into　practice.　A　thermal　model　has　been　built　based　on　the　actual　structures　and　materials　of　the　packaged　device　and　the　assembled　converter　in　order　to　evaluate　the　effectiveness　of　different　thermal　performance　improving　methods.　The　simulation　results　of　the　improved　design　demonstrate　the　possibility　of　further　increasing　the　switching　frequency　for　the　converter　while　maintaining　the　temperature　of　the　device　under　125　oC.　Finally,　the　corresponding　experiments　have　been　conducted　to　validate　the　simulation　results　using　the　fabricated　stack-die　devices.　The　presented　simulation　method　and　result　successfully　provide　a　guidance　of　thermal　design　improvement　for　the　high-voltage　cascode　GaN　HEMT　device　in　the　stack-die