In this study, the radioresistance of wild type ,Deinococcus radiodurans, KD8301 and the factors affecting the radioresistance were investigated. KH3111 which was a DNA repair mutant of KD8301 (Zeji Du, 1998) was used to be compared with KD8301. ,Deinococcus radiodurans, was discovered by Anderson ,et al, (1956) in X —ray sterilized canned meat that was found to have undergone spoilage. This bacterium and other species of this genus share extreme resistance to ionozing radiation and other agents that damage DNA. Wild type KD8301 and its sensitive mutant KH3111 were irradiated with ,60,Co γ—ray at the dose range 0.5~10kGy. Dose — survival fraction curves were made and the radio resistances were determined by LD99. The relative contents of DNA in cells were measured by Fluorescence Spectrophotometry (Freedman and Bruce, 1971) . The results indicated that wild type KD8301 possesses more radoresistant than its mutant KH3111, LD99 were 9.5kGy and 2.4kGy respectively. KD8301 grown at exponential phase showed a decreased resistance to radiation, and the LD99 was 5. 1kGy. No differences of DNA/protein in cells were found between the exponential phase and the stationary phase. The results could be concluded that wild type KD8301 possesses remarkable radioresistance, but this ability was decreased or disappeared after mutation (in KH3111) . None DNA relative content other than the growthstages were determinant factors of radioresistance in ,Deinococcus radiodurans., This results were different from other report (Dickie N ,et al, 1990) . The cellular mechanisms might be the defferences of the bacterium cell morphology between the exponential phase and the stationary phase. Recently, the mutation site of KH3111 which was mutated chemically from wild type KD8301 was identified (Zeji Du, 1998) . One base pair changed in the novel gene ,pprA, which was isolated from KD8301 genomic DNA. This point mutation was confirmed to be responsable for the sensitivity of KH3111 to ,γ,—ray and other DNA—damaging agents.