We report that treating indium tin oxide (ITO) source and drain electrodes with ozone or oxygen (O-2) plasma enhances the hole injection from these electrodes to pentacene semiconductor, thereby improving the performance of pentacene thin-film transistors. Synchrotron radiation photoelectron spectroscopy results showed that the ozone and O-2 plasma treatments of the electrodes increased their work functions by about 0.4 and 1.0 eV, respectively, compared to the as-deposited ITO electrode. These increases in work function were found to lower the hole injection barrier, producing remarkable increases in the field-effect mobility.