The influence of laser light irradiation at the surface plasmon frequency of the gold nanoparticles suspended in 2-propanol is examined by the UV-vis absorption spectral measurement and also by a transmission electron microscope. It has been found that the laser light irradiation on the colloidal suspension causes coagulation or dispersion of the particles depending on the particle concentration of the suspension. To investigate the cause of this finding, we have measured the zeta-potential of the particles by an electrophoretic light scattering method. This measurement suggests that the dispersion phenomenon of flocculates is due to the increment in the S-potential of the particles which prevents mutual access of negatively charged particles. The photochemical reaction, in which the gold particles obtain electrons from a solvent molecule, is suggested to be responsible for this process.