We investigated the changes in the organic transistor characteristics of poly(3-hexylthiophene-2,5-diyl) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) thin films upon high-energy gamma-ray irradiation. Devices based on both organic semiconductors showed a lower field-effect mobility and electrical current levels after gamma-ray irradiation. The threshold voltage of P3HT transistors shifted negative, while that of PCBM-based devices shifted positive. These changes in electrical performance of the devices can be utilized as a gamma-ray sensing elements.