Ionizing radiation causes various types of DNA lesions, such as strand breaks, oxidative base lesions and, in particular, biologically relevant complex damage known as cluster damage sites, which consist of two or more elemental lesions within one or two helical turns of DNA. The clustered DNA damages would be less readily repaired than isolated lesions and finally induce serious genetic change of a living cell. We present here the evidence that the yield of DNA lesions and clustered damage strongly depends on the LET of He ion beam. Hydrated plasmids were irradiated with He ions at LETs of 19, 63 and 121keV/µm. The yield of prompt single strand breaks (ssbs) gradually decreased with increasing LET, whereas the yield of prompt double strand breaks (dsbs) showed a peak at 63 keV/µm. Base lesions and clustered damage were revealed as strand breaks by postirradiation treatment of the DNA with base excision repair proteins endonuclease III (Nth) and formamidopyrimidine-DNA glycosylase (Fpg). Fewer enzymatically-induced ssbs and dsbs were observed as the LET increased. These results suggest that the clustering of damage becomes more intense with increasing LET. Studies with carbon ions are now in progress.