Abstract: Laminar mixed convection heat transfer of supercritical CO₂ in miniature circular tubes with different inclination angles and under constant heat flux heating condition was determined through numerical simulation. The distribution and variation of the cross-sectional temperature, axial velocity, secondary flow, the top generatrix heat transfer coefficient, the circumferential wall temperature and the Nusselt number were analysed by using the FLUENT software for different tube inclination angles, and the relative secondary flow kinetic energy was introduced to quantitatively express the intensity of the secondary flow. The results show that the top fluid temperature is higher than the bottom’s, the bottom circumferential Nusselt number is higher than the top’s, the distribution of axial velocity is not centrosymmetric and the velocity reaches peak value in the lower side of the tube center axis. The buoyancy-induced secondary flow first increases, then decreases, and reaches the maximum near the entrance. The top generatrix heat transfer coefficient reaches peak value near the critical temperature and is much higher than the bottom generatrix’s. The influence of buoyancy on convection heat transfer is obtained according to the horizontal pipe buoyancy criterion.