The　carbon　dioxide　(CO2)　trans-critical　cycle　is　widely　used　in　refrigerating　systems,　air　conditioning,　and　heat　pumps.　Heat　transfer　behaviors　at　subcritical　and　supercritical　pressures　play　essential　roles　in　trans-critical　thermohydraulic　systems.　In　past　decades,　boiling　heat　transfer　at　subcritical　pressure　and　‘single-phase＇　convective　heat　transfer　at　supercritical　pressure　have　been　broadly　discussed;　however,　evaluation　of　the　heat　transfer　difference　of　CO2　between　supercritical　and　subcritical　pressures　remains　lacking.　In　this　study,　experimental　investigations　are　conducted　to　explore　the　heat　transfer　characteristics　of　the　vertical　flow　of　CO2　in　a　wide　pressure　range　(6.2　MPa–8.0　MPa)　within　a　circular　tube　(inner　diameter　=　5　mm).　The　range　of　operating　conditions　includes　a　mass　flux　(G)　between　0　and　190　kg/m2s　and　a　heat　flux　(q)　of　up　to　105　kW/m2.　Parametric　effects　(e.g.,　heat　flux,　inlet　temperature,　and　mass　flux)　within　these　experimental　conditions　are　also　analyzed.　Measurement　results　show　that　the　inner-wall　temperature　at　subcritical　pressure　is　higher　than　that　at　supercritical　pressure　when　a　boiling　crisis　occurs.　Closer　to　the　critical　pressure,　a　boiling　crisis　occurs　more　easily　at　lower　heat　fluxes,　but　the　corresponding　temperature　peak　declines.　Higher　temperatures　are　measured　farther　from　critical　pressure,　indicating　that　overheating　induced　by　boiling　heat　transfer　at　subcritical　pressure　is　far　more　significant　than　that　governed　by　the　deterioration　of　convective　heat　transfer　at　supercritical　pressure.　Additionally,　a　qualitative　analysis　of　physical　mechanisms　governing　observed　heat　transfer　differences　is　performed,　confirming　variations　in　fluid　thermophysical　properties　and　a　bubble-like　flow　pattern.　©　2020