Optimization of the interface treatment and pentacene deposition condition was performed to improve the field effect mobility of organic thin film transistors. The full factorial design of experiment methodology was applied sort out the individual effects and interactions of the process parameters. The field effect mobility of the pentacene layer increased from 0.01 cm 2/(Vsec) to 0.3 cm2/(Vsec) after the optimization process. The resulting TFT showed an on/off current ratio of 2 × 106 and a threshold voltage of -0.3V. These values are believed to be sufficient to drive liquid crystal displays and electrophoretic-type electronic paper displays. The surface morphology investigated by atomic force microscopy confirmed that the improved alignment of the pentacene molecules by the interface modification was the key process to enhance the field effect mobility and the resulting TFT performance.