The biosynthesis and accumulation of resveratrol in grape may be regulated by genetic and environmental factors [i.e., ultraviolet-C (UV-C) irradiation]. However, there is a lack of research on the biosynthesis and accumulation of resveratrol in grape leaves responding to UV-C irradiation at sequential developmental stages. In this study, leaf resveratrol concentration during different developmental stages of three grape cultivars, Jingxiu, Beifeng, and MRH3, under natural conditions, and of ‘Beifeng’ leaves exposed to UV-C irradiation was investigated. Results showed that resveratrol synthesis was related to the developmental stages under natural conditions. In young leaves, resveratrol concentration was low, but it increased continuously during leaf development, mainly as piceid forms. UV-C irradiation greatly stimulated resveratrol synthesis in ‘Beifeng’ leaves, and young leaves at 20 days were most sensitive to the irradiation. The total resveratrol in the skin of UV-C irradiated berries is about 19 times higher than that of the control, mainly in the forms of trans-resveratrol (trans-res)/cis-resveratrol (cis-res). These results provide a basis for increasing resveratrol concentration by artificial means.