Gold nanoparticles (AuNps) were fabricated in a poly(vinyl alcohol) (PVA) film using the photochemically generated benzophenone ketyl radical and PVA radical by laser irradiation as a reducing agent. The measurements of the surface plasmon band of AuNps indicated that AuNps continued growing in the PVA film for several hours or days after the laser irradiation. The formation process of AuNps in the PVA film was investigated by using laser flash photolysis and UV-vis absorption spectroscopy. Additive doping ( formic acid or sodium 2-mercaptoethanesulfonate) in the PVA film dramatically accelerated or inhibited the formation rate of the AuNps, respectively. The doping of formic acid accelerated the formation rate of the AuNps by a factor of 10-20. On the contrary, doping of 2-mercaptoethanesulfonate inhibited the formation of AuNps. The mechanisms of the acceleration and inhibition were investigated by using laser flash photolysis. The effects of additives on the formation process of AuNps are discussed.