In this work, the authors investigated the apoptotic activities of Fe3O4/Ag nanocomposite biosynthesised by Spirulina platensis extract against MCF-7 (human breast cancer cells). The physico-chemical properties of prepared Fe3O4/Ag nanocomposite were studied by different spectroscopic methods. To evaluate the in vitro cytotoxic effect, MCF-7 cells were treated with different concentrations of Fe3O4/Ag nanocomposite and examined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide (MTT) assay. Moreover, apoptotic effects were also studied by Hoechst 33258 staining, caspase 3 activation assays, and annexin V-fluorescein isothiocyanate (FITC) and propidium iodide staining. Microscopic observations of Fe3O4/Ag nanocomposites indicated approximately spherical shape and small particles in the size range of about 30-50 nm. The MTT assay result revealed that the Fe3O4/Ag nanocomposite causes a dose-dependent cell proliferation reduction in MCF-7 cells (IC50 = 135 μg/ml). Regarding to the Annexin V/PI staining result, the increase percentage of apoptotic cells (28.09%) was detected as compared to untreated cells. According to the caspase assay, Fe3O4/Ag nanocomposite enhances caspase 3 level. Furthermore, in vitro anti-cancer activity of the nanocomposite was performed by Hoechst 33258 staining method. The proposed data suggest that Fe3O4/Ag nanocomposite may be an effective agent for the inhibition of breast cancer cells at in vitro level.