The recent interest in the development of nanomaterials has led researchers to the study of their electrical properties and the applications that they may have as insulating materials. One of these applications is the use of nanofluids as electrical insulation of power transformers. It has been reported that the dielectric properties of insulating liquids in which small amounts of nanoparticles have been dispersed are, in some cases, superior to those of the base fluids. Although these materials are promising, and their application could lead to advantages for the transformer design and reliability in the future, more research is necessary to evaluate different combination of materials under a wider range of experimental conditions. In this paper, a research on the lightning impulse breakdown voltage of mineral oil and several Fe3O4-based nanofluids is presented. Fluids prepared with different concentrations of nanoparticles were subjected to impulse lightning voltages considering both positive and negative polarities. As shown in this work the positive impulse breakdown voltage of the liquids showed improvements of up to 50%; in the case of the negative impulses not significant improvements were obtained.