Colloidal Fe-V-O nanoparticles prepared as carbon nanotube (CNT) growth catalysts were precisely size-classified by fractional precipitation. Furthermore, the classification ability was improved by the fractional precipitation after ligand exchange process, which allowed us to obtain narrower size distributions of nanoparticles. CNTs were grown from the nanoparticles in order to investigate the dependence of diameter distribution of CNTs on that of nanoparticles. The results show that the diameter distribution of CNTs grown from classified nanoparticles was narrower than that of CNTs grown from as-prepared nanoparticles.