A three-dimensional field-effect transistor incorporating vertical channels of organic semiconductor is developed to realize both high on current and high on/off ratio. A multi-block structure is built to compose metal-insulator- semiconductor layers on series of its horizontally elongated sidewalls, so that the space availability for the field-induced carriers is essentially multiplied as compared with conventional planar-type field-effect transistors. An organic thin film of dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene is vacuum deposited for the semiconductor channel on the vertical micro-structures of Si/SiO2. Pronounced field-effect performance is realized with maximum current of 8 mA in an area of 500 μm square, as the result of extremely large ratio of channel width divided by length and fairly high carrier mobility of 0.75 cm2/Vs. The result already demonstrates that more than sufficient current density for matrix-controlling devices is driven by a-few volt with the use of the three-dimensional organic transistor, indicating its availability in such applications as flat-panel displays.