Objective To exlplore the adverse effect of formaldehyde on DNA in isolated mice testicular cells. Methods The testicular cells were exposed to formaldehyde of different concentrations(0, 25, 50, 75, 100 ?mol/L), comet assay was applied to detect DNA breaks. Results Formaldehyde induced DNA breaks at 10 ?mol/L, 25 ?mol/L and 50 ?mol/L (P0.05). The critical concentration at which formaldehyde could induce DNA strand breaks of testicular cells was about 10 ?mol/L, peak point was near 50 ?mol/L. Conclusion Formaldehyde may cause DNA damage in isolated mice testicular cells in various degree.