The forced laminar and turbulent heat transfer characteristics of nanofluids in small smooth tubes were simulated using two kinds of multiphase-flow models. The simulated results were compared with the experimental results and the traditional predicting equations. The multiphase-flow models include the Euler-Euler model, of which one is the mixture model and the other is Eulerian model, respectively. The effects of various parameters such as Reynolds number and mass concentration of nanoparticles on the heat transfer characteristics were investigated and discussed in each model. The results show that little deviation exists between the simulated results and traditional predicting  equations for low concentration nanofluid. While non-traditional fluid characteristics occur and increase with the increase of the nanoparticles concentration of nanofluid, the effect of multiphase flow comes out when concentration reaches a certain value. Besides, the simulated results using special multiphase-flow models are closer to the experimental data than that of the traditional equations.