This paper investigates mechanism of tribological behavior of carbon nanotubes by using universal tribometer-II and molecular dynamics simulations. The experiment results indicate that multi-walled carbon nanotubes film with mixed acid treatment has better surface quality and less impurities but higher friction than pristine carbon nanotubes film. The reason is that mixed acid treatment introduces carboxyl group and more defects which increases dangling bonds of carbon nanotubes. Breaking of dangling bonds increases friction force when sliding and shearing occurs. Molecular dynamics simulation of shearing between silicon surfaces and single-walled carbon nanotube bundles without defect shows low lateral forces because only van der walls force exists between silicon surfaces and carbon nanotubes owing to no dangling bonds. The result is consistent to the conclusion inferred from experiment. Therefore excellent performance is expected when carbon nanotubes treated with little defects are used as lubricant or addictives.