Background and Aims Carbon nanotubes are novel materials attractive for use in many technical applications.Numerous studies have been performed to unravel their characteristic physical and electromechanical properties.However, their high length to diameter ratios mean they are fiber shaped, and thus have been compared to asbestos.In vivo toxicity studies of multi-walled carbon nanotubes (MWCNT) are very limited.Carbon nanotubes have a strong tendency to aggregate into microscpic bundles, therefore, characterization of nanoparticle is needed.We controlled MWCNT agglomerate in nano-level to obtain a stable dispersion in the instillation liquid.We evaluated the biological effects of MWCNT after single intratracheal instillation to rats.