We performed first-principles calculations on the magnetism of single-walled ZnO nanotube with zigzag structure. An ideal ZnO nanotube is found to be non-magnetic. Neutral oxygen vacancies introduce zero magnetism. Neutral zinc vacancies could bring about large local magnetic moments in the nanotube, locating mainly on the neighboring O-loops. A concentration of 6.67% neutral zinc vacancies could lead to a ferromagnetic state. A single carbon substitution for oxygen also produces large local magnetic moments. Substituting a couple of carbon atoms for oxygen atoms might result in a non-magnetic state, or produce magnetic moments with ferromagnetic coupling for an excited state. (C) 2010 Elsevier B. V. All rights reserved.