Abstract: In this research, thermal boundary layer in the forced convective heat transfer of nanofluids in fully developed laminar flow in a circular tube is considered in which thermal boundary layer grows. In thermal boundary layer temperature increases gradually. By analysis of conservation laws, it is concluded that the density of the base fluid has important role in the thermal boundary layer because both nanoparticle volume fraction and axial velocity of the nanofluid depend on it. Also to obtain temperature profile, the energy equation does not require to couple with momentum equations despite velocity has radial and axial distribution.