Breast cancer (BC) is an important cause of mortality globally. The current study aims to determine the anti-cancer effects of green tea polyphenols (PP) and epigallocatechin-3-gallate (EG), either individually or encapsulated in nanoparticles (NPs), on two different human breast cancer cell lines in vitro. The proliferation of MCF-7 and MDA-MB-231 cell lines was evaluated using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT), and apoptosis was examined using flow cytometry. We also measured entrapment efficiency (EE) using dialysis technology. The particle size (PZ) and zeta potential (ZP) of the proposed nanoparticles were investigated. The cytotoxicity test was done to examine the cytotoxic effects of polyphenols (PP) and EG on human breast cancer. Cell viability in MTT assays of cancer cells incubated with PP and EG encapsulated in nanoparticles showed significant cytotoxicity; besides, we determined the half-maximal inhibitory concentration (IC50) and fold change (FC) of the drug. The flow cytometry analysis showed a quantitative analysis of different degrees of apoptosis. Our results proved that PP NPs had the highest cytotoxic effects at 100 μM with 9.39+0.47 and 10.76+0.53 % of MCF-7 and MDA-MB-231 cell viabilities, respectively. The MCF-7 is more sensitive than the MDA-MB-231 to PP and EG NPs. PP NPs are highly effective against late apoptosis in MCF-7 (13.6%) compared to late apoptosis in MDA-MB-231 (11.8%). In conclusion, introducing PP NPs and EG NPs could have great potential against resistant human breast cancer cells via the mediation of cell death-mediated apoptosis.