The incidence and mortality of breast cancer are growing which indicates the inefficiency of the current chemotherapy drugs. Due to the anticancer potential of Zn and Ag and the magnetic feature of iron oxide, in this work, we synthesized ZnFe2O4-Ag nanocomposite using Chlorella vulgaris and investigated its anticancer effect on breast cancer cell line. Physicochemical characterization was performed by FT-IR, XRD, SEM, TEM, VSM, EDS mapping, UV, and zeta potential assays. Cell cytotoxicity and apoptosis frequency were studied by the MTT and flow cytometry assays. Also, cell cycle analysis, Hoechst staining, and measuring ROS (reactive oxygen species) level were performed. The synthesized particles were almost spherical with a size range of 14-52 nm. The FT-IR and XRD assays confirmed the proper synthesis of the particles and VSM analysis showed that particles had magnetic property and the maximum saturation magnetization was 0.8 Emu/g. Also, the EDS mapping of the nanocomposite showed the Zn, Fe, O, and Ag elements. The MTT assay showed that the 50% inhibitory concentration (IC50) of ZnFe2O4-Ag for breast cancer and normal cells were 28 and 154 µg/mL, respectively, and the nanocomposite had stronger anticancer activity than cisplatin (IC50 = 84 µg/mL). Flow cytometry analysis showed that the exposure to the nanocomposite induced cell apoptosis by 77.5% and significantly induced ROS generation. Also, treating breast cancer cells with the nanocomposite induced cell cycle arrest and apoptotic features, including chromatin condensation and fragmentation. In conclusion, ZnFe2O4-Ag nanocomposite synthesized by C. vulgaris could suppress the proliferation of breast cancer cells by the generation of oxidative stress, apoptosis induction, and cell cycle arrest.