In this study, mixed convection heat transfer properties of nanofluids in minichannels were investigated by dispersing SiO2 nanopaticles into pure water. In the study, nanofluids prepared in three different volumetric ratios of 0.25%, 0.75%, and 1.25% were investigated experimentally under laminar conditions in circular horizontal minichannels with diameters of 1.21 mm, 1.5 mm, and 1.9 mm. For the minichannels with the diameters of 1.21 mm, 1.5 mm, and 1.9 mm, increasing the particle volume ratio from 0.25% to 1.25% provided a heat transfer enhancement compared to water by about 11-31%, 7-29%, and 9-27%, respectively. Increasing the minichannel diameter from 1.21 mm to 1.5 mm and from 1.5 mm to 1.9 mm caused an enhancement of the Nusselt number in the range of 6.9-13.9% and 3.3-9.3%, respectively. Summarizing the results, the natural convection/forced convection ratio and natural convection effect on total heat transfer increased at a constant Grashof number value when the volumetric ratio of nanoparticles increased. In contrast to the particle volumetric ratio, an increase in the minichannel diameter was found to reduce the natural convection/forced convection ratio and the natural convection effects on total heat transfer at a constant Grashof number value.