This paper presents experimental and theoretical determination of the effective thermal conductivity of three magnesium oxide (MgO) nanoparticles of different sizes dispersed in glycerol. The glycerol-based nanofluids were prepared at volume fractions ranging from 0.5% to 4% and no surfactant. The nanoparticles were dispersed and deagglomerated for two hours using an ultrasonic probe. The effective thermal conductivity of nanofluids was measured from 20 ˚C to 45 ˚C using a thermal conductivity analyser. The experimental results show an increase in the thermal conductivity of MgO-glycerol nanofluids with increasing volume fraction of nanoparticles. The thermal conductivity ratio is unaffected as the temperature increases. In the given volume fraction and temperature range, the thermal conductivity ratio of MgO-glycerol nanofluids decreases with increasing particle size. The obtained experimental data were also compared with some existing theoretical and empirical models that may work for glycerol-based nanofluids. The comparison of experimental data with these available models shows that the data does not agree with the models. Therefore, a new empirical correlation was developed for the MgO-glycerol nanofluids.