Copper phthalocyanine (CuPC) has been used as the buffer layer for source and drain electrodes to improve the performance of organic thin-film transistors. The mobility was improved by almost onefold after the modification. By incorporating with CuPC, the contact resistance was reduced, deduced from the line-transfer method. The higher hole current observed in the hole-only diode after adding CuPC further confirms the improvement of hole-injection efficiency. It is concluded that the device improvement is attributed to the reduction of contact resistance, which resulted from the Fermi level pinning at the induced gap states at the AuCuPC interface.