Chemical species during growth of carbon nanotubes (CNTs) in direct current-plasma-enhanced chemical vapor deposition were studied in detail using C3H4-NH3 and CO-NH3 mixtures through optical emission spectroscopy. In the C3H4-NH3 system, the relative intensities of CN (388.3 nm) and CH (431.4 nm) decreased and that of C-2 (436 nm) increased, leading to sp(2) graphization into the CNT structure, leading to improvement of field emission property of CNTs. In the CO-NH3 system, the trend is completely reversed. Attributed to the balance of kinetics involved, CNTs could be grown most effectively under specific conditions. Based on these results, we suggest the growth mechanism and the correlation between morphologies and field emission properties of CNTs in our system. (C) 2003 American Vacuum Society.