Carbon nanotube was used as a preventing agent to prepare Ni/carbon nanotube composite nanocoatings. Effect of carbon nanotube content in electrolyte on average grain size, microstructure, microhardness and antiwear properties of the Ni/carbon nanotube composite nanocoatings was investigated. The results indicate that average grain size and microstructure of the Ni/carbon nanotube composite nanocoatings are mainly controlled by carbon nanotube content in electrolyte. The Ni/carbon nanotube composite nanocoating has an average grain size of about 16 nm as the carbon nanotube content in electrolyte is 4 wt%. The wear behaviour and microhardness of the Ni/carbon nanotube composite nanocoatings is closely related to the average grain sizes and microstructures of the Ni/carbon nanotube composite nanocoatings. The excellent antiwear and mechanical properties of the Ni/carbon nanotube composite nanocoatings are attributed to the small grains and compact microstructures of the Ni/matrix composite nanocoatings caused by carbon nanotube used as a preventing agent.