The insulation property of transformer oil is a critical factor affecting the safe operation of power equipment. The nonwoven polypropylene (PP) electret films used for purification of transformer oil were treated by fluorination and DC corona charging, and the oil filtration properties were investigated. The results show that the C-H bonds in polypropylene can be replaced by C-F bonds during fluorination process and the mean diameter of fibers decreases while the roughness of the fibers increases, which represents the super-hydrophobicity. When the fluorinated PP films were corona charged and applied to transformer oil filtration, it was observed that the aged oil filtered by the modified films got a better recovery of insulation properties in comparison to that of oil filtered by original PP films. There was a great increase in the breakdown voltage. Furthermore, the content of moisture, the relative dielectric permittivity and loss factor decreased significantly.