An organic semiconductor device includes a carrier, a source, a drain, an organic semiconductor single-crystalline channel layer, an organic insulation layer and a gate. The source and the drain are disposed on an upper surface of the carrier. The source and the drain are disposed in parallel and a portion of the carrier is exposed between the source and the drain. The organic semiconductor single-crystalline channel layer is disposed on the upper surface of the carrier and covers a portion of the source, a portion of the drain and the portion of the carrier exposed by the source and the drain. The organic insulation layer covers the carrier, the source, the drain and the organic semiconductor single-crystalline channel layer. The gate is disposed on the organic insulation layer and corresponds to a position of the portion of the carrier exposed by the source and the drain.