We report the viscosity measurement of water-based nanofluids containing CuO spherical nanoparticles of about 29 nm in diameter. Using an innovative experimental procedure and a microfluidic “Viscometer Rheometer On a Chip”, (mVROCTM) viscosity data were collected for several nanoparticle volume fractions and a shear rate ranging up to 20,000s-1. The effect of low nanoparticle volume fraction and temperature on nanofluids viscosity was considered and discussed in this work. The stability and dispersion state of nanofluids were also studied from rheological measurements and nanoparticle tracking analysis. The possible effects due to the temperature hysteresis behavior of aggregated nanofluids under cooling and heating cycle is reported as well.