Multiwalled carbon nanotubes (CNTs) grown by thermal chemical vapor deposition (CVD) in a tube furnace at atmospheric pressure are reported. The CNTs were synthesized at 750 degrees C using C2H2 as the carbon source and a mixture of Ar/H-2 (80 vol % :20 vol %) as a carrier gas, with NH3 serving as a processing reagent. The catalytic properties of nickel (Ni) particles are strongly affected by the CVD process parameters and this effect is reflected in the morphologies and field emission behaviors of the as-grown CNTs. The flow rate ratio of NH3 to C2H2 is found to be critical to the formation of vertically aligned CNTs, which demonstrated better field emission characteristics than the randomly oriented nanotubes. The successful synthesis of vertically aligned CNTs at atmospheric pressure in a tube furnace is beneficial for large area mass production such as flat panel field emission displays at lower cost. (c) 2006 American Vacuum Society.