Abstract： The natural convection heat transfer and flow characteristics of nanofluids in an enclosure were numerically simulated using the multiphaseflow model and single phase model respectively. The simulated results were compared with the experimental results from the published papers to investigate the applicability of these models for nanofluids. The multiphaseflow model is mixture model. The effects of various parameters such as Rayleigh number, Grashof number and volume concentration of nanoparticles on the heat transfer and flow characteristics were investigated and discussed. Comparisons of the central velocity profiles between nanofluid and water for various Gr numbers were studied as well. In addition, streamlines contours and isotherms for different volume concentration were analyzed. The results show that a great deviation exists between the simulated results based on single phase model and experimental data on NuRa heat transfer curve, which indicates that the simulation based on single phase model and NS equations cannot reflect the heat transfer characteristic of nanofluid, while the simulated results using multiphaseflow model have good agreement with the experimental data of nanofluid, which means that the multiphaseflow model is more suitable for the numerical study of nanofluid. Moreover,  nanoparticles can significantly enhance the motion of fluid in the enclosure, which is in favor of strengthening the energy transfer in fluid. Besides, nanofluids also show the characteristic of strengthening the convection in the enclosure.