Heat transfer deterioration is numerically studied for supercritical fluids flowing upward in circular tubes at high heat fluxes and low mass fluxes. The simulations are conducted with Shear Stress Transport (SST) k-omega turbulent model in commercial software Fluent 15.0. Both water and CO2 are simulated and the results are consistent well with the experiments. It is found that there are two peaks of wall temperature when the heat transfer deterioration occurs, the first peak is narrow and sharp while the second peak is lower and broader. The mechanism of two wall temperature peaks is analyzed in details based on radial distributions of velocity and turbulent kinetic energy at different axial positions. It is found that the mechanism of the first peak is quite different from the second peak. The first peak is caused by buoyancy effect, which flattens the velocity distribution in the near wall region and leads to the reduction of turbulent kinetic energy and the impairment of heat transfer in the near wall region. For the second peak, it is the shear stress that flattens the velocity distribution in the main flow region and leads to the reduction of turbulent kinetic energy, which again impairs the heat transfer and causes the second peak. (C) 2017 Elsevier Ltd. All rights reserved.