Abstract: Objective To study the effects of pendimethalin and clethodim on DNA damage of mice bone marrow cells in vitro and in vivo.Methods The comet assay was used to detect the DNA damage induced by pendimethalin and clethodim at different doses.Results Compared with the dimethyl sulfoxide(DMSO) group,the DNA damages in groups treated with 10,20 mg/L of pendimethalin were significantly different(P0.05,P0.01),and clethodim(20 mg/L)could affect DNA of mice bone marrow cells significantly in vitro(P<0.05).The results showed that the groups treated with 500,1000 mg/kg·bw of pendimethalin were significantly different compared with the DMSO group in vivo(P0.05,P0.01),while the clethodim(250-1000 mg/kg·bw)did not induce the DNA damage significantly(P<0.05).Conclusion Pendimethalin could cause DNA damage in mice bone marrow cells in vitro and in vivo significantly,while clethodim could not.