Experiments were conducted to investigate the characteristics of water with copper oxide nanoparticle suspensions flowing through circular stainless steel tubes. The internal diameters of the tube are 0.68 mm, 1.01 mm and 1.28 mm. The mean diameter of the nanoparticle is 50 nm. The mass fractions of copper oxide nanoparticles in the experiment are 0.02, 0.04 and 0.06. The mass fraction of surfactant SDBS is 0.02. The result of the flow experiment shows that the pressure drop of copper oxide nanoparticle suspension is higher than that of the water. Pressure drop will increase if the mass fraction of the suspension increases. The critical Reynolds number of transition from laminar to turbulent flow, which appears earlier than in normal size tube case, would increase with the increasing of diameter.