Abstract: A junction structure of an organic semiconductor device including an organic semiconductor layer, a conductive layer and a modifying layer is provided. The modifying layer is formed between the organic semiconductor layer and the conductive layer, wherein the modifying layer includes an inorganic compound or an organic complex compound. An organic thin film transistor including a gate, a source/drain, a dielectric layer, an organic semiconductor layer and at least a modifying layer is also provided. The gate is electrically isolated from the source/drain. The dielectric layer is disposed between the gate and the source/drain. The organic semiconductor layer is disposed between the source and the drain. The modifying layer is disposed between the organic semiconductor layer and the source/drain, wherein the modifying layer includes an inorganic compound or an organic complex compound.