Abstract: In this paper, the transient cooling heat transfer performance of supercritical CO₂ is simulated using the one-dimensional and three-dimensional models. The transient heat transfer performances predicted by the two models are compared and the buoyancy effect is discussed. The results show that the response periods predicted by the 1-D model have a large deviation from that predicted by the 3-D model; the difference between the two models is enlarged when the fluid temperature is closed to the critical temperature; and the tube diameter is large. The buoyancy effect is considered to be one of the main reasons for the difference. The Richardson number is used to quantify the buoyancy effect. It is found that the difference between the response periods predicted by the 1-D model and the 3-D model is significant if the Richardson number is larger than 0.1.