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BACKGROUND The use of nanoparticles synthesized by the green method to treat cancer is fairly recent. The aim of this study was to evaluate cytotoxicity, apoptotic and anti-angiogenic effects and their expression of involving genes, of zinc oxide nanoparticles (ZnO-NPs) synthesized with Carob extract on different human breast cancer cell lines. METHODS ZnO-NPs synthesized using the extract of Carob and characterized with various analytical techniques. The MCF-7 and MDA-MB231 cells were treated at different times and concentrations with ZnO-NPs. The cytotoxicity, apoptosis and anti-angiogenic were examined using a series of cellular assays. Expression of apoptotic genes (Bax and Bcl2) and anti-angiogenic genes, vascular endothelial growth factor (VEGF) and its receptor (VEGF-R) in cancer cells treated with ZnO-NPs were examined with Reverse transcription-quantitative polymerase chain reaction (RT-qPCR). The antioxidant activities of ZnO-NPs evaluated by ABTS and DPPH assay. RESULTS Exposure of cells to ZnO-NPs resulted in a dose-dependent loss of cell viability. The IC50 at 24, 48 and 72 hours were 125, 62.5 and 31.2µg/ml respectively (p<0.001). ZnO-NPs treated cells showed in fluorescent microscopy that ZnONPs are able to upregulate apoptosis and RT-qPCR revealed upregulation of Bax (p<0.001) and downregulation of Bcl-2 (p<0.05). ZnO-NPs increased VEGF gene expression while decreasing VEGF-R (p<0.001). The antioxidant effects of ZnO-NPs were higher than control group and were dose dependent manner (p<0.001). CONCLUSION ZnO-NPs synthetized using Carob extract have the ability to eliminate breast cancerous cells and inhibit angiogenesis so could be used as anticancer agent.