The role of plasma functionalization of multi-walled carbon nanotubes for ammonia gas sensors was investigated. Plasma functionalization of multi-walled carbon nanotubes with maleic anhydride was carried out with various durations. The sensing properties were tested at room temperature. Active material of gas sensor was investigated by scanning electron microscopy, energy-dispersive X-ray spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. It has been shown that formation of functional groups on the surface of carbon nanotubes led to the increase in ammonia sensor response by 2-4 times. The formation of plasma coating on the surface of multi-walled carbon nanotubes is also accompanied with growth of sensor resistance. The increase of functionalization duration induces the rise of O:C from 0.28 to 0.335 and distortion of shape of I-V curves of active material of ammonia gas sensor.