Multi- and single-walled carbon nanotubes (MWCNTs and SWCNTs) were prepared by the chemical vapor deposition from ethanol-saturated Ar gas onto catalytic substrates at 1073 K under magnetic fields of up to 10 T. MWCNTs were aligned in parallel to magnetic fields more than 4 T in the growth process even at 1073 K. Also, the formation of metallic SWCNTs of around 1.0-nm diameter was promoted over 4 T. These results demonstrate that magnetic fields may control the growth of CNTs.