The interactive effect of 0.4 W/m２ UV-B radiation and 0.4% NaCl stress on DNA in the primary leaves of two mung bean cultivars (Phaseolus raditus L. cv. Zhonglü-1 and Qindou-20) was studied. The results showed that (1) mung bean cultivar “Zhonglü-1" is more tolerant to both UV-B radiation and NaCl stress than cultivar “Qindou-20". NaCl stress reduced UV-B sensitivity of “Zhonglü-1", but did not change UV-B sensitivity of “Qindou-20". (2) Treatment of the two cultivars by either UV-B or NaCl alone or by both stresses together resulted in a decrease in DNA hyperchromicity in the two mung bean cultivars. The extent of the decrease in cultivar “Zhonglü-1" was lower than that in cultivar “Qindou-20". Decrease in DNA hyperchromicity in the two mung bean cultivars under both UV-B and NaCl stresses together was lower than that under NaCl stress alone. (3) UV-B-induced cyclobutyl pyrimidine dimers (CPD) accumulation in cultivar “Zhonglü-1" was lower than that in cultivar “Qindou-20"; NaCl stress could reduce the UV-B-induced CPD accumulation in cultivar “Zhonglü-1", but did not change that in cultivar “Qindou-20". (4) All stress treatments caused a drop of DNA content in two mung bean cultivars but with cultivar “Zhonglü-1" showing stronger responses. These results indicate that UV-B radiation can induce not only the formation of CPD on the same DNA strand, but also the formation of cross-linking between the two strands of DNA and the decrease in DNA content. It is also suggested that the different UV-B sensitivity between two mung bean cultivars under either NaCl stress or no NaCl stress might be mainly caused by the differences in CPD accumulation and cross-linking between the two strands of DNA.