The aim of this research was to evaluate the in vitro potential of 177 Lu-labeled gold nanoparticles conjugated to cyclo-[RGDfK(C)] peptides ( 177 Lu-AuNP-c[RGDfK(C)]) as a plasmonic photothermal therapy and targeted radiotherapy system in MCF7 breast cancer cells. Peptides were conjugated to AuNPs (20 nm) by spontaneous reaction with the thiol group of cysteine (C). After laser irradiation, the pres - ence of c[RGDfK(C)]-AuNP in cells caused a significant increase in the temperature of the medium (50.5 °C, compared to 40.3 °C without AuNPs) resulting in a significant decrease in MCF7 cell viability down to 9 %. After treatment with 177 Lu-AuNP-c[RGDfK(C)], the MCF7 cell proliferation was significantly inhibited.