The application of Electrostatic Force Microscopy (EFM) was studied in analyzing the corona-charged polypropylene fiber. Electrostatic force gradient images were obtained from the phase shifts with the varied bias voltages applied to the EFM cantilever by the noncontact scans. A mathematical expression to model the EFM phase shifts as a function of the applied tip bias voltages was introduced. EFM analysis was used to test the hypothesis that the solventinduced efficiency deterioration of electret filter media originates from charge deterioration. EFM investigation produced evidence that exposure to isopropanol in the liquid phase affected the electrostatic charges on fibers. Exposure to organic solvents in liquid phase is thought to increase chain and charge mobility in polypropylene fibers, thereby reducing the electrostatic charge and the particle capture ability.