Disclosed is a method of manufacturing an organic semiconductor thin film, including preparing semiconductor ink containing a solvent, a low-molecular-weight organic semiconductor and a high-molecular-weight organic semiconductor and forming an organic semiconductor thin film vertically phase-separated by applying the semiconductor ink on a substrate through a bar-coating process using a bar. In the bar-coating process of the invention, the semiconductor ink blend is used, and the gap between the substrate and the bar is adjusted, thus controlling vertical phase separation. Also, the speed of the bar, the gap of which is adjusted, is regulated, thus controlling crystal growth, whereby the uniformity of the thin film is improved and thus a high-quality organic semiconductor crystalline thin film having a large area can be manufactured in a continuous process. Also, a flexible organic semiconductor transistor, having high stability and high charge mobility, can be provided using the organic semiconductor thin film.