Irradiation of a pulsed Nd:YAG laser at 532 nm to gold particles of less than 50 nm in aqueous solution was found to cause the shape change and size reduction of the particles. Typically, the nonspherical gold particles between 20 and 50 nm in diameter disappeared, whereas the number of gold particles of spherical shape less than 10 nm increased. The size reduction ceased after 5 min irradiation. The maximum diameter in the size distribution decreased to ca. 10 nm when the laser fluence was increased up to nearly 800 mJ cm-2. The temperature of the gold particles was estimated from the absorbed laser energy by the particles and was found to rise as high as the boiling point of gold; these results were supported by the measurements of the blackbody radiation from the particles. The shape change and size reduction are considered to occur through melting and vaporization of the gold particles. The high temperature, which causes melting and vaporization, is a result of the strong absorption of the laser energy by the particles and the low heat transfer to the surrounding water.