Gold colloidal nanoparticles were prepared by the liquid laser ablation of a gold metal plate in water and also by the citrate reduction of HAuCl(4).4H2O. The gold colloidal nanoparticles with the plasmonic band strongly absorb light, which is converted to the photothermal energy. This photothermal energy gives a cytotoxic effect on epithelial carcinoma cells. Interestingly, we found that the size and shape of the nanoparticles are changed by light during the photothermal process in vitro. The cervical carcinoma cell line (HeLa cell) was incubated with the colloidal gold nanoparticles and then exposed to continuous visible light at 400-600 nm with UV- and heat-cutoff filters. The distinct cell-killing effect was observed by this procedure. In the absence of the gold colloidal nanoparticles, only a small amount of cells were photothermally destroyed.