To study the relationship between changes of microbial ATP in four kinds of murine tissues and the postmortem interval (PMI),healthy SD rats were sacrificed and their muscles,livers,spleens and kidneys were sampled at different postmortem intervals. The concentration of microbial ATP was detected using bioluminescent assay and the data was statistically analyzed. The concentration of microbial ATP in muscle increased with PMI time. The peak appeared at the 7th day after death,and at the 10th day,microbial ATP in muscle tissue increased again. In intal organs,the peaks of microbial ATP were observed at the 8th day after death and the level decreased during 8-10 d. The differences in microbial ATP concentration in liver,spleen and kidney were not statisticallysignificant. During day 0 to day 9 after death,the correlation was best between PMI and microbial ATP in muscle. With PMI as the independent variable,the cubic polynomial regression equation was Y=0.02X3-0.166X2-0.666X+13.412 (R2=0.989,P＜0.01). In intal organs,the best correlation was found between PMI and microbial ATP during day 0 to day 10. With PMI as the independent variable,the cubic polynomial regression equation was Y=0.016X3-0.127X2-0.809X+13.324 (R2=0.986,P＜0.01). There existed high correlations between PMI and microbial ATP concentration in rat tissues.Since only a small amount of tissue was needed for the detection and the sample was not affected by self-decomposition,the method may extend the time range of PMI estimation.