This paper is aimed at studying the influence of conductive (Fe 3 O 4 ) and insulating (Al 2 O 3 and SiO 2 ) nanoparticles at various concentrations on the AC dielectric strength of MIDEL 7131 synthetic ester. The Normal and Weibull distribution functions are used to analyze the breakdown voltages with 1, 10, and 50% risk probabilities estimated. It is shown that the breakdown voltage values for most of investigated nanofluids obey both Normal and Weibull distributions. The addition of considered nanoparticles enables to improve the dielectric strength of synthetic ester. However, this improvement depends on the concentration, size and nature of nanoparticles. For a given type and size of nanoparticles, there is an optimal concentration that allows us to reach the highest value of breakdown. On the other hand, the magnetic/conducting nanoparticles give the best improvement of dielectric strength. Indeed, with Fe 3 O 4 (50 nm) the enhancement is of 48% while it is of 35 and 25% with Al 2 O 3 for nanoparticles of 13 and 50 nm, respectively, and it is of 32% with SiO 2 (50 nm).