Abstract: Heat transfer experiments of supercritical water in 2×2 rod bundles were conducted based on Chinese Supercritical Water Cooled Reactor(CSR1000) fuel assembly design. The experimental parameters were as follows: the system pressure 23~25 MPa, the mass flow flux 680~1400 kg/(m2?s), and the heat flux 174~968 kW/m2. The experimental results indicated that the heat transfer performance of the bundles reduced as heat flux increasing and mass flow flux decreasing while kept insensitive to the pressure as which varied from 23 MPa to 25 MPa. Moreover, it was found that the heat transfer performance of 2×2 rod bundles was influenced by both the differences from bulk flow to boundary layer in thermal physical properties and the non-uniform of flow and heat transfer in different sub-channels. Based on the experimental data, a satisfactory heat transfer correlation of 2×2 rod bundles was obtained, and about 88.9% of the experimental points had deviations from the correlation within ±25%.