Cancer is one of the major causes of death in the world and despite many years of research, the treatment of cancer is still a problem. Epidemiological observations and laboratory studies have indicated anticarcinogenic potential of garlic, which has been traditionally used for various human diseases around the world. In this study the cytotoxicity of garlic extract against three malignant cancer cell lines including gastric (AGS), breast (MCF-7) and colon (HT-29) and a nonmalignant cell line (L929) were evaluated by the MTT assay. The results of this study reveal MCF-7 and AGS cells were sensitive to garlic extract. Despite a many reports on inhibitory effects of garlic on cancer cell line, our data showed these effects are tumor specific and dose dependent. Further studies on animal models and humans are needed to clarify the important molecules and their mechanisms.