The cracking behavior in polystyrene (PS) nanocomposite thin films prepared by a solution casting method was investigated experimentally, and the formation mechanism of the cracks was clarified. A glass substrate was blade-coated with a toluene solution containing polystyrene and surface-modified nanoparticles prepared by the supercritical hydrothermal synthesis method; then, toluene was evaporated by a directional surrounding gas flow. In the case of nanocomposite thin films with oleic acid-modified nanoparticles, it was found that regularly spaced cracks were generated, and that the crack spacing was particularly affected by the partial vapor pressure of the solvent in the surrounding gas. On the other hand, there were no cracks in the thin films with decanoic acid-modified nanoparticles. In addition, the spatial structures of the thin films were observed by scanning electron microscopy and transmission electron microscopy. These observations revealed that a uniform layer of oleic acid-modified nanoparticles was formed at the surface of the thin film. Therefore, it was concluded that the cracks were formed by the tensile stresses due to the capillary force acting on the solvent meniscus between the nanoparticles in the surface layer during solvent evaporation.