Abstract: With the background of the low flow rate heat transfer in case of the natural circulation in the reactor, the mixed convection was experimentally studied in a 5×5 rod bundles. The experiment was performed at 6 MPa, the mass flow rate from 25 to 150 kg/(m²·s), the heat flux from 25 to 300 kW/m², Reynolds number from 1000 to 30000, Buoyancy parameter from 2×10^-7 to 3×10^-3. It is concluded that with the increasing of the Buoyancy parameter, the heat transfer in rod bundles was impaired firstly and then followed by enhancement. It is observed that in the rod bundle channel, the influence of the buoyancy force occurs at Bo*=3.5×10^-6. Even when Reynolds number is higher than 15000, the heat transfer could be impaired by the buoyancy force. An empirical correlation used to calculate mixed convection heat transfer in rod bundles was proposed based on the experimental data.