Aims: To investigate effect of metallic nanoparticles, silver (AgNPs) and gold nanoparticles (AuNPs) asantitumor treatment in vitro against human breast cancer cells (MCF-7) and their associated mechanisms. Thiscould provide new class of engineered nanoparticles with desired physicochemical properties and may presentnewer approaches for therapeutic modalities to breast cancer in women. Materials and Methods: A humanbreast cancer cell line (MCF-7) was used as a model of cells. Metallic nanoparticles were characterized usingUV-visible spectra and transmission electron microscopy (TEM). Cytotoxic effects of metallic nanoparticles onMCF-7 cells were followed by colorimetric SRB cell viability assays, microscopy, and cellular uptake. Natureof cell death was further investigated by DNA analysis and flow cytometry. Results: Treatment of MCF-7 withdifferent concentrations of 5-10nm diameter of AgNPs inhibited cell viability in a dose-dependent manner, withIC50 value of 6.28μM, whereas treatment of MCF-7 with different concentrations of 13-15nm diameter of AuNPsinhibited cell viability in a dose-dependent manner, with IC50 value of 14.48μM. Treatment of cells with a IC50concentration of AgNPs generated progressive accumulation of cells in the S phase of the cell cycle and preventedentry into the M phase. The treatment of cells with IC50 concentrations of AuNPs similarly generated progressiveaccumulation of cells in sub-G1 and S phase, and inhibited the entrance of cells into the M phase of the cell cycle.DNA fragmentation, as demonstrated by electrophoresis, indicated induction of apoptosis. Conclusions: Ourengineered silver nanoparticles effectively inhibit the proliferation of human breast carcinoma cell line MCF-7in vitro at high concentration (1000 μM) through apoptotic mechanisms, and may be a beneficial agent againsthuman carcinoma but further detailed study is still needed.