Multi-walled carbon nanotubes (MWCNTs) have been grown, simultaneously at different temperatures (600 °C, 700 °C, 725 °C, and 750 °C), by thermal chemical vapour deposition (CVD) of acetylene (C2H2) gas, in ammonia (NH3) atmosphere. The samples have been characterized morphologically by scanning electron microscopy (SEM) and a structurally by Raman spectroscopy. These analyses show that, when the temperature of substrate increases, the CNT average diameter and the density distribution decrease, and the CNTs are less defective. However there is a temperature limit (725 °C) for CNT growth. We have grown CNTs onto Si3N4/Si substrates with Pt electrodes, in order to realize the gas sensing devices. The CNTs grown at 725 °C, exhibit a fast response and a high sensitivity to NO2 gas.