This　paper　presents　a　numerical　comparison　of　the　flow　and　conjugate　heat　transfer　characteristics　about　the　internal　cooling　of　a　nozzle　guide　vane　with　three　kinds　of　coolants,　including　air,　steam　and　mist/steam.　Five　radial　cooling　channels　are　established　inside　the　vanes.　The　Reynolds-averaged　Navier　Stokes　equations,　coupled　with　a　fully-developed　Shear　Stress　Transport　(with　gamma-theta　transition)　turbulent　model,　are　adopted　and　solved.　Different　coolant　mass　flow　rates　are　examined.　Different　initial　mist　diameters　and　mist　concentrations　are　numerically　calculated.　The　mist　tracks　in　five　internal　channels　and　the　cooling　effectiveness　at　the　mid-span　are　obtained　and　compared　among　different　initial　mist　diameters　and　mist　concentrations.　The　turbulence　kinetic　energy　and　heat　transfer　coefficient　inside　the　internal　channel　are　used　to　further　investigate　the　effects　of　the　droplet　size.　The　mean　cooling　effectiveness　of　the　vane　outer　surface　is　obtained　at　different　coolant　mass　flow　rates.　Results　show　that　the　mist/steam　cooling　has　a　best　cooling　performance　compared　with　that　of　the　air　and　steam.　The　influence　of　the　mist　concentration　is　much　smaller　than　the　initial　mist　diameter　on　the　mists　evaporation.　The　mists　can　evaporate　entirely　at　a　relative　small　initial　diameter　and　the　evaporation　distance　increases　about　two　times　with　the　mist　mass　flow　rate　increases　from　1%　similar　to　5%　coolant　mass　flow　rate.　With　a　same　mist　concentration,　the　faster　the　mists　evaporate,　the　higher　the　cooling　effectiveness　is　obtained.　Under　a　certain　coolant　mass　flow　rate,　the　amplification　of　the　mean　cooling　effectiveness　decreases　with　the　increase　of　the　mist　concentration.　With　the　increase　of　the　coolant　mass　flow　rate,　the　differences　of　the　mean　cooling　effectiveness　among　different　mist　concentrations　become　larger.