The difficulties encountered in Linear Polarization Resistance(LPR) measurements are discussed. A method for determining Differential Polarization Resistance(DPR) is proposed, Its principle is explained, and a DPR measuring device based on this principle is set up. By this technique, the theoretical errors arising from linearizing approximation in LPR measuring techniques will not exist, and other problems encountered in LPR measuring techniques such as how to choose measuring frequency and polarization value properly for a given corroding metallic electrode system will disappear. The effect of nonfaradaic current on results measured may be ignored by this measuring technique, so it is possible to perform the measurement rather rapidly and thus to trace the corrosion potential closely. The DPRs as measured by this technique have been compared with the theoretical PRs and LPRs obtained from polarization curves for Fe in 4 different solutions.