Disclosed is a method of manufacturing an organic semiconductor thin film for an organic thin film transistor from a blend of organic semiconductor/insulating polymer. The organic semiconductor thin film is configured such that organic semiconductor nanofibrils are dispersed in the form of a network in the insulating polymer layer. This organic semiconductor thin film is formed by dissolving the blend of organic semiconductor/insulating polymer in a marginal solvent of the organic semiconductor or mixed solvent thus preparing a blend solution, which is then applied while adjusting the solubility of the solution. An organic thin film transistor using the organic semiconductor thin film is also provided. The blend thin film of organic semiconductor/insulating polymer containing only about 3 wt % of the organic semiconductor exhibits electrical properties equal to those of a thin film composed exclusively of an organic semiconductor. The insulating polymer acts as a protective layer of the organic thin film transistor, thus obviating a need for forming the protective layer.