The characteristics of carbon nanotube obtained by thermal chemical vapor deposition (CVD) process were investigated as a function of ammonia (NH3) gas in hydrocarbon gas, growth time, growth temperature, and Fe catalyst thickness. Fe catalyst was prepared on Si by DC magnetron sputtering with thickness of 5-40 nm and pre-treated with NH3. CNTs were then grown with ammonia-acetylene gas mixture by thermal CVD. From our results, it was found that NH3 in C2H3 gas positively acted to improve CNT quality. The density and diameter as well as quality of CNTs were strongly affected by catalyst shape and thickness.