Contact effects have been investigated in fully printed p-channel organic thin film transistors with field effect mobility up to 2 cm 2/Vs. Electrical characteristics of the organic thin film transistors, with channel length< 200 μm, are seriously influenced by contact effects with an anomalous increase of the contact resistance for increasing source-drain voltage. Assuming that contact effects are negligible in long channel transistors and using gradual channel approximation, we evaluated the current-voltage characteristics of the injection contact, showing that IV characteristics can be modeled as a reverse biased Schottky diode, including barrier lowering induced by the Schottky effect.