The application of Electrostatic Force Microscopy (EFM) on characterizing a corona charged polypropylene fiber by phase measurements was studied. Electrostatic force gradient images were obtained from the phase shifts with varied EFM tip bias voltages at a constant tip-to-sample distance, and with varied tip-to-sample distances at a constant tip bias voltage. Mathematical expressions were introduced to interpret the phase measurements in terms of the tip bias voltage and the tip-to-specimen distance. The phase measurements from EFM appeared feasible to provide the qualitative information of electrical properties of charged polymeric materials. The fit of mathematical expressions to the experimental measurements was discussed with regards to the effect of tip geometry on the interaction between the tip and the specimen.