The Raman spectra of calf thymus DNA in aqueous solution before and after irradiation in two UV regions (UVA, 320-340 nm; UVA-UVB, 280-400 nm) were investigated. Damage to the molecular structure in part was found in the DNA solution after UVA-UVB irradiation. The bands near 811 (ยฑ7) and 832 (ยฑ7) cm -1 appeared in all Raman spectra after UV irradiation. This result indicates that DNA in solution has a mixture of A-and B-form conformations under UV irradiation. In this experiment, all the main characteristic Raman peaks appeared at almost the original position after UVA irradiation, except for some bands corresponding to deoxyribose phosphate. This indicated that serious changes to the molecular structure did not occur in the DNA solution after UVA irradiation. The spatial structures of the DNA main chain backbone and phosphoric acid backbone were almost unchanged, and the bands corresponding to adenine also appeared at almost the original sites after UVA-UVB irradiation. The bands at 949 and 1445 cm -1 corresponding to deoxyribose shifted and were obviously hypochromic, and the bands assigned to thymine showed a large shift and decline after 5 h of UVA-UVB irradiation. Hence the pyrimidine bases and deoxyribose were likely first damaged when the calf thymus DNA in aqueous solution was irradiated by UVA-UVB.