This study deals with the fabrication of novel highly-sensitive NH3 sensor based on a macrocyclic complex (5,7,7,12,14,14-hexamethyl-l,4,8,11-tetraazacyclo-tetradeca-4,11-diene)nickel(II) iodide complex. Its gas-sensing properties were first investigated to NH3, H2S, NO2, SO2, water vapour (H2O) and some organic solvent vapors. The sensors exhibited a linear response to NH3 in a wide concentration range from 2.4ppm to 1.9% at room temperature. No responses were observed to H2S, NO2, SO2, water vapour (H2O) and organic solvent vapors. The investigation proved that the response to NH3 was variable with the operating voltage. The optimum operating voltage was 5V. The response and recovery time were about 40s and 25s, respectively. Since this sensor has an excellent selectivity and sensitivity, it appears promising for the practical application in determination of ammonia.