The structures of four new flavone derivatives, kuwanon A, B, C and oxydihydromorusin were isolated from the benzene extract of the root bark of the cultivated mulberry tree (a variety of Morus alba L.), and were shown to be I, II, III and IV, respectively, on the basis of spectroscopic and chemical evidences. Photooxidative cyclization did not occur with I but with II leading to the formation of a hydroperoxide possessing a dihydrooxepin ring. III was cyclized on photooxidation as II and was converted to morusin (V) by treating with DDQ. On hydration with methanolic hydrochloric acid, V was converted to IV.