The triangular Au nanoparticle array of size 37 nm was fabricated on a quartz substrate using nanosphere lithography. By performing the Z-scan method with femtosecond laser (800 nm, 50 fs), the optical nonlinearity of the Au nanoparticle array was determined. The results showed an intensity-related two-photon absorption saturation processes. As the excitation intensity increases, the nonlinear absorption changes from two-photon absorption to saturated absorption, while the nonlinear refraction exhibits self-defocusing effect. The one- and two-photon figures of merit, W=7.5 and T=0.12, were obtained, validating a high efficiency nonlinear material for all-optical switching.