Colloidal Au less than 20nm was prepared in an aqueous solution of HAuCl4 by irradiation with a near-infrared femtosecond laser. The absorption peak around 526nm in the absorption spectra and the peaks 2θ = 38.3°and 2θ = 24° in the x-ray diffraction (XRD) patte indicated the formation of Au colloid. Nonlinear optical properties of Au colloid were investigated by using the Z-scan technique with 8ns pulses at 532nm. The transition characteristics from self-defocusing to self-focusing and from saturable absorption (SA) to reverse saturable absorption (RSA) were observed for the Au colloid.