The study aims at exploring mechanisms of the biological effect induced in human hepatoma SMMC-7721 cells irradiated by high LET heavy ions. Carbon (12C6+) and argon (40Ar+) ions from Heavy Ion Re- search Facility at Lanzhou (HIRFL) and pulse-field gel electrophoresis were used in this study. The results showed that in the given dose range the DSB induction increased linearly with dose and the slope of dose-effect curve of the 40Ar+ ions with higher LET was bigger. Meanwhile we found that when dose increased there were different varieties of the distribution of DSB fragments induced by ions with different LET and the quick repair to DSB was completed in 30 minutes and then came the slow repair phase for two kinds of irradiation. For the 40Ar+ ions irradiation with higher LET the amount of repaired DSB was larger than 12C6+ ions irradiation in both repair phases and there was lit- tle distinction between the residual DSB induced by two kinds of irradiation. The results approve the linear relation- ship between DSB induction and dose and display the influence of ion kind to the slope of dose-effect curve. As well as the results affirm the non-random distribution of DSB induced by heavy ions and the two phase mode of DSB re- pair and show the synchronously introduced impact to the distribution of DSB by LET

ion kind and dose. Slow repair and misrepair should be given attention when research on repair to DSB induced by heavy ions is involved.