The development of nanotechnology is currently undergoing rapid, especially on high-voltage equipment. One of them is nanofluids as a replacement oil insulation medium for transformers developed by mixing nanoparticles with oils. One type of conductive nanoparticles, Fe_3O_4, has been extensively studied and tested for increasing breakdown voltage. However, when the transformers are used continuously, the dielectric properties of the oil insulation will decrease. Moreover, a cellulose paper insulation on a transformator will inevitably deteriorate as well, resulting in insulation paper particles becoming contaminants in the transformator oil insulation. besides the amount of cellulose concentration, the aging time of nanofluids is also observed. In this paper, the percentage of cellulose contamination in Fe_3O_4 nanofluids at 0%, 10%, 25%, 50%, and 100% as well as aging times 0 hour and 250 hours compared. The sedimentation test method and particle size analyzer test are performed to determine the distribution of particles as well as the solubility of nanofluids and cellulose. The results showed that higher cellulose concentrations resulted in a decrease in the breakdown voltage strength of Fe_3O_4 nanofluid isolation. It can be stated that cellulose paper has impacted the insulator strength of nanofluid oil.