The　objective　of　this　paper　is　to　develop　a　criterion　to　predict　the　onset　of　heat　transfer　deterioration　(HTD)　for　supercritical　CO2　heat　transfer.　A　new　mechanism　is　proposed　by　assuming　supercritical　pseudo-boiling.　Before　bulk　fluid　reaches　pseudo-critical　temperature　(Tpc),　the　tube　cross-section　contains　a　vapor　layer　and　a　liquid-like　fluid.　The　saturation　temperature　interface　is　defined　at　T　=　Tpc,　inside　and　outside　which　are　a　vapor　layer　(T　＞　Tpc)　and　a　subcooled　liquid　(T　＜　Tpc).　The　subcritical　boiling　number　is　extended　to　supercritical　“boiling”　number,　defined　as　SBO　=　qw/(Gipc),　where　qw,　G　and　ipc　are　heat　flux,　mass　flux,　and　CO2　enthalpy　at　Tpc,　respectively.　SBO,　by　coupling　the　density　ratio　between　“liquid”　and　“vapor”　represents　the　competition　between　vapor　expansion　induced　momentum　force　and　inertia　force.　The　experiment　of　supercritical　CO2　heat　transfer　is　performed　in　a　10.0　mm　inner　diameter　tube,　covering　ranges　of　P　=　7.5–21.1　MPa,　G　=　488–1600　kg/m2s　and　qw　=　74–413　kW/m2.　Surprisingly,　the　onset　of　HTD　is　found　to　occur　at　a　critical　SBO　which　is　5.126×10−4　and　the　critical　heat　flux　is　expressed　as　qCHF=5.126×10−4Gipc.　It　is　shown　that　our　new　criterion　is　also　suitable　for　other　experiments　reported　in　literature,　providing　a　general　guidance　to　design　and　operate　S–CO2　heaters　to　avoid　HTD.　©　2018　Elsevier　Masson　SAS