Photodynamic cancer therapy (PDT) had already proved its effectiveness in vitro but the search for more effective photosensitizers is encouraged and ongoing. Gold nanoparticles (AuNPs), have been shown to be good drug delivery agents, but in this work AuNPs were investigated as photodynamic agents for PDT. AuNPs were synthesized and characterized by means UV-Vis spectroscopy and transmission electron microscopy (TEM). Photodynamic effects of AuNPs in MCF-7 cells were evaluated using trypan blue, adenosine triphosphate (ATP) luminescence and lactate dehydrogenase (LDH) membrane integrity for cell viability, proliferation and cytotoxicity, respectively. AuNPs had a peak absorption at 540 nm, spherical in shape and were successfully taken up by MCF-7 cells. As a result of light activation or not, cell damage was observed and AuNPs modified into dendrimer-entrapped gold nanoparticles (AuDENPs), which only yielded effects in a light dependent manner. AuNPs is not a suitable photodynamic agent but its modified AuDENPs can be essential in improving the efficiency of PDT.