Epoxidation of imperatorin (1a) with m-chloroperbenzoic acid (mcpba) under the irradiation of gamma-ray gave a mixture of epoxide 3a and dioxofuran 4a. Whereas, alloimperatorin (1b) under the same condition obtained the epoxide 3b as a sole product. On the other hand, it was successfully epoxidized xanthotoxin (1c) with mcpba under the same condition as above. In addition, visnagin (2a) was epoxidized to give a mixture of epoxides 5a, 6a. While, khellin (2b) gave the epoxide 5b, no other products were observed.