An ultrathin composite film containing cationic tetra-pyridine perylene tetracarboxylic acid diimide (TPP-I) and an anionic tetra-sulfuric perylene tetracarboxylic acid diimide (TSP-Na) has been fabricated by the electrostatic layer-by-layer self-assembly technique. UV/Vis spectra 14 showed a continuous and uniform deposition process of TPP-I/TSP-Na. The film structure was characterized by the small-angle X-ray diffraction measurement and atomic force microscopy image. We investigated the third-order nonlinearity of the ultrathin film sample at 532 nm. The film exhibited strong saturable nonlinear absorption. The nonlinear absorption coefficient and refractive index of the film sample are -5.0 x 10(-7) mW(-1) and -1.2 x 10(-14) m(2)W(-1), respectively. (c) 2007 Elsevier B.V. All rights reserved.