For field emitters as an electron source of traveling wave tube microwave power amplifiers, field-emission properties of multiwalled carbon nanotubes (MWNTs) grown in situ onto an electrically conducting substrate were systematically characterized. MWNTs grown on a TiN-coated Ta substrate with NH3 plasma pre-treatment exhibited the best field-emission property. The maximum current density and corresponding total emission current were 9.4 mA/cm(2) and similar to5 mA at 18.8 V/mum, respectively. These enhanced field-emission properties are caused by the highly conducting buffer layer (TiN), the optimum metal substrate (Ta) that produces high field-emission current, and the control of site density of MWNTs by NH3 plasma pre-etching. Details on the correlation between the field-emission properties and the morphological parameters will be discussed. (C) 2004 American Vacuum Society.