Laminar mixed convection of a NANOFLUID consists of water and Al2O3 in a horizontal tube has been studied numerically. Continuity, momentum and energy equations are discreted with the volume control approach and the second-order accuracy and coupling pressure and velocity with the SIMPLE algorithm is done. To include free convection, variation of fluid density with temperature varies according to the Boussinesq equation. Two-phase mixture model has been used by using of effective properties for modeling of two-phase NANOFLUID flow. Meshing has been done with the organization, and next to the walls, boundary layer mesh has been created. Investigating the results obtained in laboratory conditions indicates the acceptable accuracy of numerical simulations. Simulation has been performed in two states of constant the Richardson number and heat flux, steady state and the variation of the heat transfer coefficient and the Nusselt number are presented in graphical form in various conditions. The results show that, in the case of constant heat flux, the choice of suitable formula for the effective thermal conductivity coefficient will have a significant effect on the accuracy of the obtained results.