Carbon nanotubes (CNTs) were grown on 25 nm Ni coated Si substrate by dielectric barrier discharge (DBD)-type plasma enhanced chemical vapor deposition (PECVD) in downstream. The temperature was 973 K and the mixture gases were methane, hydrogen and water. The contribution of water concentration on the growth of carbon nanotubes was investigated. Scanning electron microscopy (SEM) results showed that the grown CNTs in CH4/H2 gas mixure had chain-shaped structure and the diameter was about 40—90nm. With the addition of proper amount of water, the diameter of CNTs became uniform, and the surfaces of nanotubes were fully covered with nano-sized node-like structures. When more water was added, branched carbon nanotubes were synthesized. Optical emission spectra of the plasma during synthesis of carbon nanotubes were in-situ detected for the purpose of understanding the water effect.