The large-size domain and continuous para-sexiphenyl (p-6P) ultrathin film was fabricated successfully on silicon dioxide (SiO2) substrate and investigated by atomic force microscopy and selected area electron diffraction. At the optimal substrate temperature of 180 degrees C, the first-layer film exhibits the mode of layer growth, and the domain size approaches 100 mu m(2). Its saturated island density (0.018 mu m(-2)) is much smaller than that of the second-layer film (0.088 mu m(-2)), which begins to show the Volmer-Weber growth mode. The characteristic of liquid-like crystal of p-6P monolayer film and the adequate diffusion of p-6P molecules dominate the formation of large-size domain. The coalescence of large-size domains offers the possibility to grow high-quality p-6P monolayer film which provides excellent substrate for weak epitaxy growth of phthalocyanine compounds.