The challenge with current conventional gas sensors which are operating using semiconducting oxides is their size. After the introduction of nanotechnology and in order to reduce the dimension and consequently the power consumption and cost, new materials such as carbon nanotubes (CNTs) are being introduced. From previous works and characterization results, it was proven that the CNTs based gas sensor has better sensitivity, selectivity and faster response time in compared with semiconducting oxides based gas sensors. As in this work, a fabrication and successful testing of an ionization-based gas sensor using aligned Multiwalled CNTs (MWCNTs) as sensing element is discussed, in which MWCNTs array and Al film are used as anode and cathode plates respectively with electrode separation ranging from 80 μm to 140 μm. Aligned MWCNTs array was incorporated into a sensor configuration in the gas chamber for testing of gases such as argon, air, and mixed gas of 2 H 2 in air. Obtained results show that among the three gases, argon has the lowest breakdown voltage whilst air has the highest value and the breakdown voltage was found to decrease as the electrode spacing was reduced from 140 μm to 80 μm for all three gases. © 2011 American Institute of Physics.