Carbon nanotubes (CNTs) are well known for their exceptional thermal, mechanical and electrical properties. For many CNT applications it is of the foremost importance to know their health hazards related to their exposure. Normal human bronchial epithelial cells (BEAS-2B) has been used for assessment the cytotoxicity of SWCNT (Diameter--1.2-1.5 nm) and DWCNT (Diameter--1.3-5 nm). Clear interference of CNTs with conventional in vitro cytotoxicity assays (MTT, NRU and LDH) dye was found which was confirmed by acellular system. However morphological changes and flow cytometry showed the characteristics of cytotoxicity. Thus our study showed that there is a need of appropriate method for the assessment of cytotoxicity of CNT.