This paper presents a comparative study of the dielectric strength of mineral oil-based Al 2 O 3 nanofluids and mineral oil. The measurements are achieved in a sphere-tosphere electrodes arrangement according to IEC 60156 specifications. It is shown that Al 2 O 3 nanoparticles enhance the insulating performances of mineral oil and the degree of enhancement depends on the concentration and size of nanoparticles. The smaller the nanoparticles are, the higher the effect on dielectric strength of nanofluid is. For nanoparticles of about ten nm, the optimal BDV is obtained with a concentration of 0.05 g/L; the improvement is of 76%. While with nanoparticles of 50 nm, the optimal BDV is obtained with a concentration of 0.3 g/L; in that case, the improvement is of about 69%.