Rod-shaped gold nanoparticle (gold nanorod: GNR) are a typical anisotropic gold nanoparticle. We found that photochemical synthesis of GNRs in a micellar solution of hexadecyltrimethylammonium bromide (CTAB) was triggered by photoreactions of ketones. Photoexcitation of gold ions and silver bromide clusters ((AgBr)n) in the absence of ketones did not produce GNRs. The initial products of the photoirradiation were probably ketyl radicals, which then initiated reactions to form GNRs. GNRs formed in darkness, if UV light irradiated the reaction solutions for a few minutes. Thus, the photochemical products catalyzed GNR-formation. In the dark reactions, silver ion (Ag+) played important role. Transmission electron microscope observations indicated that the growth of GNRs in the longitudinal direction was accelerated by the addition of Ag+; however, in the transverse direction, Ag+ suppressed the growth of GNRs. This tendency was the same as that in the electrochemical synthesis of GNRs. It was shown that the photochemical and electrochemical syntheses of GNRs involved the same processes for the anisotropic growth of gold nanocrystals.