The impact of introducing SiO₂ nanoparticles on the thermal conductivity (TC) values of various basefluids is presented in this paper. Microstructural properties of SiO₂ nanoparticles were investigated using transmission electron microscope. Nanofluids were produced by dispersing SiO₂ nanoparticles in water andcommercialized engine coolant (CEC) at different mass concentrations in a range of 0.01–1%. Dynamic light scattering, visual examination, absorbance in the visible region, and zeta (ζ) potential analysis were introduced to investigate the size distribution and stability of nanoparticles in the dispersed phase. The results show that nanofluids had good dispersion stability. Enhancement in the TC of nanofluids was improved by increasing mass concentration and temperature. The TC improvement was more in CEC than in water. The highest enhancements in TC values of nanofluids were 21.08% and 1.89% with CEC and water respectively at a mass concentration of 1% and temperature of 65 °C. Finally, a new correlation of nanofluids TC has been proposed.