Abstract: The damaged lymphocytes and Hela cells were divided into control groups and supplemental groups treated with 25μl of 1μl/ml endonuclease.The results showed that the damage of DNA strand breaks of lympnocytes was much lower in control with 63.85 arbitary units(AU)than supplement group about 161.93 after 4 hour culture(P<0.05).The similar result was found in Hela cells.The DNA strand breaks were significantly increased from 19.0 in control group to 172.1 in suppiement groups(P<0.01)after 4 hour culture.It was indicated that the supplementation of endonuclease played an important role in the base incision repair and produced a lot of strand breaks from enzymic incision for DNA repair.