An irradiation test was performed for polymethylmethacrylate plastic optical fibers under gamma-ray irradiation in order to use the fiber in low-level radiation environments. Under gamma-ray irradiation at a high dose rate, only a large radiation-induced transmission loss at wavelengths less than 700 nm was observed. Under irradiation at a low dose rate, the loss was small and other two characteristic effects were observed. One is a radiation-induced loss that has peaks in the near infrared wavelength range, and the other is an improvement in the optical transmission over an entire wavelength range. The peak intensity depends on the absorbed dose of the gamma ray. Therefore, it is considered that the absorbed dose can be measured using the peak intensity. Irradiation tests were performed by changing the configuration of the fiber sample to observe the effect of a bending loss. It was found that the improvement in the optical transmission is dependent on the configuration of the irradiation sample and that the bending loss can be recovered by gamma-ray irradiation at a low dose rate.