Abstract： This paper numerically investigated the natural convection of supercritical nitrogen/argon (n(N2)∶n(Ar)=0.9∶0.1,  n is amount of substance) binary fluid in a 3dimensional rectangular cavity. The results showed that a stable thermal boundary layer forms at the bottom of the cavity during the initial heating phase, and the Piston effect plays a dominant role to accelerate the energy transport in this period. After the thermal plume breaks the boundary layer and moves upward, the natural convection occurs with the appearance of a cold boundary layer near the top. The Soret effect caused by the temperature gradient drives the component migration, and results in the Dufour effect which promotes the energy transport. This results indicate that the Soret effect and Dufour effect can promote the heat and mass transfer in the natural convection of supercritical binary fluids.