We present an experimental study of fiber optic ammonia gas sensor based on the phenomena of surface plasmon resonance working on wavelength modulation scheme. The principle of the sensor is based on the change in dielectric constant of the bromocresol purple (BCP) in the presence of ammonia gas. The sensor works at room temperature. Two different kinds of coating configurations have been considered, namely copper + BCP and silver + BCP, on the unclad portion of the fiber. The experiments have been carried out at the low concentrations (1 ppm − 10 ppm) of ammonia gas around the probe. The sensor with copper and BCP layers has greater sensitivity than sensor with silver and BCP layers. The proposed sensor has small response and recovery times.