The effect of buoyancy on sealing performance of rim seal was analyzed by using numerical method to simulate the quasi three-dimensional rim seal， based upon the Boussinesq approximation.The temperature and velocity fields were analyzed, and flow and heat transfer characteristics of subpass were discussed. The results show that the buoyancy caused by gravity almost has no effect on leakage, while the buoyancy caused by centrifugal force has driving action in radial direction on surface flow of rotating parts, which increases the flow resistance in subpass, and sealing performance of the rim seal is getting better. Under the condition of thermal boundary, the effect of buoyancy caused by centrifugal force on leakage coefficient is related to Grashof number. Effect of buoyancy caused by centrifugal force on the sealing performance of rim seal is increased along with the increasing of wall temperature and rotational speed. Along with the increasing of axial-clearance, the leakage coefficient is increased, and the effect of buoyancy caused by centrifugal force on the leakage coefficient is decreased.