One of the most important problems in the synthesis of carbon nanotubes is the problem of controlling their morphology, namely: length, aspect ratio, alignment, etc. Catalytic synthesis of carbon nanotubes in a suspended bed reactor allowed to study the possibilities of controlling the growth of nanotubes by introducing a certain amount of water vapor and carbon-containing materials in the reaction zone. The synthesized long carbon nanotubes were studied by Raman spectroscopy, transmission and scanning electron microscopy. We found that water concentration influences both yield and the structure of nanotubes. It is shown that the yield of centimeter-long nanotubes can be maximized at an optimum H2O/C ratio, while deviations dramatically change morphology and thickness of the nanotubes.