This study evaluated the effect of low-dose gamma irradiation (0.3–0.9 kGy) on carotenoid accumulation in Cara Cara fruit (Citrus sinensis L. Osbeck) during storage. Additionally, the irradiation-induced ROS (reactive oxygen species) response and transcriptional levels of carotenoid biosynthesis genes were investigated. Immediate irradiation caused a temporary decrease in most carotenoid levels; however, values recovered during storage and were even higher than those of the control, particularly at 0.6 kGy, especially for carotenoid precursors and carotenes. The upregulated expression of psy, zds and lcyb during majority of the storage period, coupled with the strengthened interaction among adjacent carotenoids, might contribute to the enhanced accumulation of carotenoids in irradiated fruit. A highly positive correlation was found among ROS content, total carotenoids, and upstream expression of zds and lcyb, suggesting that the enhanced carotenoid content in irradiated Cara Cara fruits might be regulated by zds and lcyb in response to ROS accumulation. Antioxidant enzymes of APX, SOD, and CAT levels were significantly correlated with ROS accumulation. In summary, gamma irradiation (0.6 kGy) elevated the levels of carotenoids and antioxidant enzymes, which were potentially modulated by ROS in response to irradiation stress.