A new way was developed to grow carbon nanotubes (CNTs) in the downstream zone of high pressure dielectric barrier discharge plasma. Using Ni coated Si as the catalyzed substrate, CH4 as the carbon source, H2, NH3 and water vapor mixtures as the dilution and etching gas, CNTs were grown under temperatures of 630oC and 750 oC. The effects of various etching gas mixtures on the growth of CNTs were investigated respectively. The CNTs were characterized by SEM and high-resolution TEM. The results show that trace addition of Ammonia and water vapor to the plasma atmospheres can influence CNTs growth strongly. In 15% CH4 gas mixtures, CNTs can grown well within an addition of 3% NH3 which showed an improvement of CNTs quality, but no product was founded when adding the NH3 content over 8%. On the contrary, in 15% CH4 and 8% NH3 gas mixtures, CNTs can grow better when adding a small proportion (2‰-6‰) of water vapor to the etching gas and meanwhile optical emission spectrometry (OES) was applied to diagnositic and explain the effects of trace water vapor on the process of CNTs growth.