A novel organometallic third-order nonlinear optical crystal: ethyltriphenylphosphonium bis(2-thioxo-1,3-dithiole-4,5-dithiolato)aurate(III) (ETPPADT) has been prepared, and large optical-quality single crystals have been grown by the solvent evaporation method. Its structural, optical and physicochemical properties are characterized by elemental analyses, UV-Vis-NIR transmission, infrared spectroscopy and thermal analysis. The thin films of ETPPADT doped into poly(methyl methacrylate) (PMMA) with a concentration of 1 wt% have been prepared using spin-coating method. The linear refractive index and the thickness of the thin films were measured by a prism coupler. The third-order nonlinear optical properties of the films have been investigated using Z-scan technique at the wavelength of 1064 nm with laser duration of 20 ps, which reveal that the composited films exhibit large negative nonlinear refractive indices about 4 x 10(-10) esu and no nonlinear absorption. The influence of thickness of the film and the incident laser intensity on the third-order optical nonlinearities have been investigated. Two figures of merit, W and T, have been calculated to be vertical bar W vertical bar congruent to 120 >> 1 and vertical bar T vertical bar=0 << 1, respectively. All the results show that ETPPADT crystal has potential application on all-optical switching. (C) 2011 Elsevier B.V. All rights reserved.