Gold nanoparticles (Au NPs) have applications to photothermal therapy because of their photothermogenic properties. The photoresponsiveness of Au NPs is largely affected by their size. The photothermogenic properties of different-sized Au NPs were investigated. The Au NPs with approximately 50 nm diameter exhibited the most efficient photothermogenic activity, which was apparent from their in situ photocytotoxicity against HeLa cells.