A novel method based on the force vs. distance curve (force curve) of atomic force microscopy was developed to measure ultrathin poly (o-phenylenediamine) (PPD) film electropolymerized on gold electrode in liquid. When 1 to 0.25 μm/s was chosen to be the rising rate of the scanner and 50% of the confidence interval as the qualifying threshold value, the thickness of the hydrated polymer film could be calculated. This result was compared with one obtained from an AFM image. A step-like electrode fabricated by a photolithographic process was used. The height difference of the electrode before and after the PPD coating was imaged in liquid, and then the real thickness, 19.6±5.2 nm, was obtained. The sample was also measured by estimating the transition range of the force curve of hydrated PPD film, and the thickness of the hydrated PPD film was determined to be 19.3±8.2 nm. However, the results calculated by integrating the electropolymerized charge for the oxidation process of o-PD was only one-third as large as it was the two previously described methods. This indicated that the structure of hydrated PPD film might have been swollen.