Ammonia (NH3) measurement method using a potentiostatic sensor was developed. NH3 emitted from a gasoline vehicle in WLTC mode was measured using the suggested method and was compared with that measured using a near-IR laser based NH3 measurement system and FTIR. It was found that the NH3 signal obtained from the method was qualitatively in agreement with those obtained from the other measurement systems. The factors leading to the difference between the NH3 concentration in exhaust measured by our new method and those measured by FTIR or laser absorption spectrometer were also investigated.