This study was undertaken to address the current deficient knowledge of cellular response to nano sized particle exposure.  The aim of this study was to investigate the in vitro responses of human bronchial epithelial cells (BEAS-2B) after exposure to nano-materials (Ag and CNT). BEAS-2B cell viability decreased with decreasing doses of Ag (diameters : 40-70 and 100 nm) and MWCNT (diameters : 4-6 and 10-15 nm) nano-materials. At the higher exposure (≥200 ug/ml), all materials induced significant cell damage. Nano Ag materials increased apoptosis and cytokine expression (IL-6 and IL-8) when over 100 ug/ml concentration. All MWCNTs weakly increased apoptosis and did not significantly increased cytokine expression. In conclusion, we suggest that Ag and MWCNT nanomaterials induce variable extents of cellular toxicity in a dose-dependent and size-dependent.