The relationship between the molecular stacking arrangement and the third-order nonlinear optical property was investigated for the evaporated thin film of bis(diphenylglyoximato)platinum(II), Pt(dpg)(2), which formed one-dimensional metal chains in the solid state. Structural control of Pt(dpg)(2) in the film was performed by changing the evaporation conditions. It was revealed that Pt(dpg)(2) formed a thermally stable red phase and a metastable green phase on a solid substrate. Their constituent ratio in the film was dependent upon the evaporation rate. The green phase, which seemed to exist only in a thin film under support of a solid substrate, was newly prepared by slow evaporation. Although it was difficult to analyze the crystalline structure of the green phase because of its rapid transformation to the red phase, absorption spectra indicated that it had shorter metal-metal distance than the red phase. The third-order nonlinear susceptibilities chi((3)) for two different modifications of Pt(dpg)(2) were evaluated by optical third-harmonic generation measurements. The chi((3)) values got larger with increase of the green phase ratio in the film. Fabrication of the green phase led to considerable improvement of transmittance and nonresonant conditions for optical nonlinearity in the visible region. Enhancement of the third-order optical nonlinearity of Pt(dpg)(2) is discussed in terms of molecular structure in the film.