Abstract The present study was aimed at evaluating the potential toxicity and the general mechanism involved in multi wall carbon nanotubes (MWCNT)-induced cytotoxicity using human embryonic kidney cell line (HEK293) cells. Two multi wall carbon nanotubes (coded as MWCNT1, size: 90–150 nm and MWCNT2, size: 60–80 nm) used in this study are MWCNT1 (produced by the electric arc method and size of the nanotubes was 90–150 nm) and MWCNT2 (produced by the chemical vapor deposition method with size of 60–80 nm). To elucidate the possible mechanisms of MWCNT induced cytotoxicity, cell viability, mitochondrial function (MTT assay), cell membrane damage (LDH assay), reduced glutathione (GSH), interleukin-8 (IL-8) and lipid peroxidation levels were quantitatively assessed under carbon nanotubes exposed (48 h) conditions. Exposure of different sizes of two carbon nanotubes at dosage levels between 3 and 300 μg/ml decreased cell viability in a concentration dependent manner. The IC50 values (concentration of nanoparticles to induce 50% cell mortality) of two (MWCNT1, MWCNT2) nanoparticles were found as 42.10 and 36.95 μg/ml. Exposure of MWCNT (10–100 μg/ml) to HEK cells resulted in concentration dependent cell membrane damage (as indicated by the increased levels of LDH), increased production of IL-8, increased TBARS and decreased intracellular glutathione levels. The cytotoxicity and oxidative stress was significantly more in MWCNT2 exposed cells than MWCNT1. In summary, exposure of carbon nanotubes resulted in a concentration dependent cytotoxicity in cultured HEK293 cells that was associated with increased oxidative stress.