Energy shortage and environment pollution are the major and large problems presently encountered by human all over the world. It is an effective way to save energy and reduce emission of polluted gas by using the nanofluids technology. There has been not a widely recognized theory which can explain flow and heat transfer of nanofluids until now. So the mechanism of flow and heat transfer of nanofluids is not clear. Considering the Brownian motion of nanoparticles in nanofluids, a mechanism model for heat transfer by heat convection is proposed based on the fractal distribution of nanoparticle. No additional/new empirical constant is introduced. The proposed fractal model for heat flux of nanofluids is found to be a function of temperature, average nanoparticle size, concentration, fractal dimension of nanoparticles, fractal dimension of active cavities on boiling surfaces and basic fluid property in pool boiling. The model predictions are compared with the existing experimental data, and fair agreement between the model predictions and experimental data is found for the cases of different nanoparticle concentrations and different average nanoparticle diameters. The analytical model can reveal the physical principles for convection heat transfer in nanofluids.