This work is devoted to find an appropriate criterion for thermal judging of nanofluidics. The importance of this issue arises from the possible misleading which asserts an existence of extra heat transfer for nanofluid compared to the base fluid, neglecting the hydraulic effects such as increasing the pressure drop. To clarify the issue, an experimental apparatus with ability of making fixed Reynolds number and constant pumping power was constructed and thermal behavior of silicon oxide/water nanofluid and distilled water are investigated in the turbulent regime. In this regard, heat transfer coefficient inside the finned air cooled heat exchanger is evaluated. Results are provided for different inlet temperatures and different nanofluid concentrations for two criteria of fixed Reynolds number and constant pumping power. Results show that the criterion of fixed Reynolds number between nanofluid and base fluid may lead to a misleading of extra heat transfer. Indeed we are obligated to increase the flow velocity to compensate the viscosity increment to make the constant Reynolds number. Hence, the existence of extra heat transfer would be comparatively related to growth of convection rather than nanoparticles effect. But it is confirmed that the criterion of constant pumping power does highly appear the real effect of nanoparticles on the heat transfer enhancement (C) 2015 Elsevier B.V. All rights reserved.