Optical nonlinearities of Au nanoparticles precipitated inside glasses induced by irradiation of a focused femtosecond laser were investigated using Z-scan technique with 8 ns pulses at 532 nm. The transformation behaviors both from self-defocusing to self-focusing and from saturable absorption to reverse saturable absorption were observed in the glass samples, which depend drastically on the irradiation power density of the femtosecond laser used. The optical nonlinearities were analyzed in detail and proved numerically by the excited-state theory of conduction band electrons. (C) 2002 Elsevier Science B.V. All rights reserved.