Multi-walled carbon nanotubes are widely used nanomaterials. Toxic effects of MWCNTs were revealed by various studies, thus prompting concerns about the effects of MWCNTs on human health and the environment. MWCNTs can be aerosolized in ambient air, and absorbed into the body through the respiratory tract. Inhaled MWCNTs can induce harmful effects on the respiratory tract and on the circulatory system. Important factors related to the toxicity of MWCNTs, such as the surface area of the nanotubes and the amount of catalytic metal included, have been revealed by various studies. These factors have an interdependent effect on MWCNT toxicity, which may differ according to the type of MWCNT. In this study, we compared the effects of purified and unpurified MWCNTs on A549, Jurkat, and THP-1 cell lines, and found that the unpurified and purified MWCNTs cause a reduction of cell viability as well as abnormal variations of cellular progression. In particular, in the case of 24 hr exposure samples, S-phase delayed pattern was appeared and the cyclin D1 was highly expressed.