P-channel pentacene field effect transistorswith a Si/SiO2/pentacene/Au structure were fabricated, and were gamma-ray irradiated with a Co60 source. The changes of the drain current ID vs. source/drain voltage VSD (ID - VSD) characteristics were measured after every 200 Gy in silicon (GySi) irradiations up to the total dose of 1200 GySi. The drain current ID continuously decreased to less than 10 % of that before irradiation after 1200 GySi irradiation. The threshold voltage Vth continuously decreased up to 800 GySi, started to saturate above 800 GySi, and recovered above 1000 GySi. The mobility m continued to decrease up to 1200 GySi. Those behaviors were explained by accumulation of positive trapped charge within the gate insulator SiO2 near the interface, continuous increase of interface traps near the interface between the SiO2 and pentacene, and build up of electrons in the channel regions. These behaviors were discussed in comparisons with previously reported results on ultraviolet (UV) light irradiation experiments on similarly structured pentacene-based transistors.