Abstract Aligned carbon nanotubes were synthesized under a combination of 20 different C 2H 2/H 2/NH 3 compositions at 700 °C using a thermal chemical vapor deposition method. Thin film Fe was used as the catalyst, which was pretreated with H 2 or NH 3 prior to the growth of carbon nanotubes. The use of different pretreatment gases results in little difference in the growth and characteristics of the carbon nanotubes except that the carbon nanotubes grown on H 2 treated catalysts have smaller diameters. The growth rate of the CNTs does not depend on the NH 3 concentration but on the ratio of NH 3/C 2H 2. There is a critical NH 3/C 2H 2 ratio that is independent of the C 2H 2 concentration and at which the peak growth rate occurs. The critical value was found to be 4.7 ± 1.2. Microstructural analysis indicates that the carbon nanotubes obtained at higher NH 3 concentrations contain defects and disorder. Field emission tests show that the carbon nanotubes exhibit a turn-on field of 2.36 V/μm and a maximum current density of 1.91 mA/cm 2. The field emission properties were found to be stable after 15 test cycles.