Laser irradiation onto silver colloidal nanoparticles dispersed in pure water was carried out at various laser intensities to modify structure of nanoparticles. At laser intensity higher than 150 mJ/cm(2), formation of wires and sheets, probably due to fusion of nanoparticles, were observed. On the other hand, at lower laser intensity (50-100 mJ/cm(2)), it was found for the first time that particles with crystalline structures, nanoprisms and nanorods, were formed.