Herceptin, the monoclonal antibody, was successfully immobilized on gold nanoparticles (GNPs) to improve their precise interactions with breast cancer cells (SK-BR3). The mean size of the GNPs (29 nm), as determined by dynamic light scattering, enlarged to 82 nm after herceptin immobilization. The in vitro cell culture experiment indicated that human skin cells (FB) proliferated well in the presence of herceptin-conjugated GNP (GNP-Her), while most of the breast cancer cells (SK-BR3) had died. To elucidate the mechanism of cell death, the interaction of breast cancer cells with GNP-Her was tracked by confocal laser scanning microscopy. Consequently, GNP-Her was found to be bound precisely to the membrane of the breast cancer cell, which became almost saturated after 6 hours incubation. This shows that the progression signal of SK-BR3 cells is retarded completely by the precise binding of antibody to the human epidermal growth factor receptor 2 receptor of the breast cancer cell membrane, causing cell death.