Experimental　investigation　was　carried　out　to　study　heat　transfer　characteristics　of　supercritical　CO2　at　low　mass　flux　conditions.　A　vertical　tube　with　an　inner　diameter　of　16　mm　was　tested　at　various　heat　fluxes　and　pressures.　The　flow　was　driven　by　mixed　convection　with　strong　buoyancy　force　owing　to　low　mass　flux　and　high　heat　flux.　The　experimental　results　shown　that　a　＇no　wall　temperature　peak＇　heat　transfer　deterioration　with　an　exactly　large　temperature　difference　between　the　tube　and　fluid　occurred　at　high　enthalpy　region,　which　was　especially　different　from　the　general　deterioration　phenomenon　with　a　wall　temperature　peak　occurred　at　normal　mass　flux　in　previous　studies.　Heat　transfer　in　low　mass　flux　was　accompanied　with　a　heat　transfer　enhancement　phenomenon　before　the　pseudo　critical　point　and　a　deterioration　after　that.　With　increasing　heat　flux,　the　deterioration　was　strengthened,　but　the　enhancement　was　little　weakened　and　moved　towards　the　low　enthalpy　region　due　to　the　combined　effect　of　buoyancy　and　specific　heat　of　fluid.　The　effect　of　pressure　on　heat　transfer　in　various　cases　was　different.　Heat　transfer　was　reduced　with　a　rising　pressure　at　low　heat　flux　cases,　but　when　heat　flux　became　larger,　heat　transfer　was　enhanced　with　the　increase　of　pressure.　The　parameters　representing　to　buoyancy　and　thermal　acceleration　effects　were　also　introduced　respectively　to　explain　the　mechanism　of　heat　transfer.　Results　shown　the　buoyancy　effect　and　the　distribution　of　large　specific　heat　fluid　were　greatly　responsible　for　the　enhancement　and　the　deterioration　in　heat　transfer.　While　the　effect　of　thermal　acceleration　on　heat　transfer　in　the　present　study　could　be　neglected.　©　2016　Association　for　Computing　Machinery　Inc.　All　Rights　Reserved.