Many of the anti-cancer compounds which are currently used have an origin in natural sources including plants. Aloe vera is one of the plants that has been used in traditional medicine for centuries to treat a variety of diseases and cancer prevention. In this study, the cytotoxic effect of Aloe vera crude extract on tumor cell lines including HL60 human acute myeloid leukemia and MCF-7 breast cancer cells was studied by cell viability assay, changes in cell morphology, and apoptosis analysis. According to the results, the treated cells with Aloe vera extract in comparison to the untreated cells exhibited significant decline in viability in a time and dose dependent manner. MTT assay showed that IC50 value of Aloe vera extract on MCF-7 cells was 0.5 mg/ml during the first 24 hours, while in this time IC50 of extract on HL 60 cells was 1 mg/ml. Also, morphological characteristics of treated MCF-7 & HeLa cells showed typical features of cell death at the morphological level such as rounding off of cells, cell shrinkage and detachment from the substrate, thus indicating that Aloe vera extract induces cell death by apoptosis. Cell death mediated by through the apoptotic pathway was also confirmed by TUNEL assay. Interestingly, Aloe vera extract did not have any significant cytotoxicity towards normal cells. Therefore, the difference in sensitivity to the Aloe vera extract between MCF-7 and HL60 cancer cells and normal cells suggested that Aloe vera extract can be used as a chemotherapeutic drug for treatment of human acute myeloid leukemia and breast cancer.