Abstract: Experiments were carried out to investigate the heat transfer of spacer grid downstream in a vertical 5×5 rod bundle. The experimental Reynolds number varies from 1 000 to 30 000, and buoyancy parameter Bo* is from 2×10^-7 to 3×10^-3. It’s observed that heat transfer of spacer grid downstream is strongly dependent on buoyancy parameters. When the buoyancy parameter is high, heat transfer of spacer grid downstream is different from convectional knowledge and shows complicate behaviors. Based on the current experimental data, a new correlation accounting for the buoyancy effect was proposed and shows good predictions.