Abstract: A method for determining the O6-methylguanine(O⁶-mG) in mouse's tissues(liver and lung) with high performance liquid chromatography tandem mass spectrometry(HPLC-MS/MS) was established.Using CD₃O⁶-mG as internal standards,the tissue samples were extracted with DNA reagent kits,went through acid hydrolysis at 80℃ and extraction with HLB solid phase extraction column,then analyzed by HPLC-MS/MS. The results showed that:1) O⁶-mG displayed good linearity in the range from 0.2 to 10 ng/mL with r of 0.9998, the limit of detection(LOD) of 0.044 ng/mL,the recoveries from 88.2% to 92.2%,and RSD from 0.96% to 2.13%.2) The concentrations of O⁶-mG in liver and lung of mice with alcoholic fatty liver were 1.25 and 2.96 times as high as that of normal mice,respectively,in 4 hours after dosing;it indicated that long-term heavy drinking might promote the carcinogenicity of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone(NNK).This method is suitable for the analysis of O⁶-mG in mouse's tissues.