Tert-butylhydroquinone (TBHQ) was tested for potential cytotoxicity and genotoxicity upon A549 lung cancer cells and Human Umbilical Vein Endothelial Cells (HUVEC). Cytotoxicity was evaluated by MTT assay and flow cytometry analysis, while genotoxicity was assessed in vitro by alkaline comet, DNA fragmentation and DAPI staining assays. TBHQ dose- and time-dependently decreased the growth of A549 and HUVEC cells. Flow cytometry analyses determined early and late apoptosis in the treated cells. Also, single strand DNA breaking has been observed through comet assay technique. In addition, morphology of DAPI stained cells and DNA fragmentation assay using gel electrophoresis showed clear fragmentation in the chromatin and DNA rings within the nucleus of cell’s treated TBHQ.