Aim To study on the mitochondrial mass change in mouse thymocyte apoptosis.Method Control and Dexamethasone(DEX) groups were set;at 6h,we studied mitochondrial mass changes by NAO and Mitotracker Green(MG) staining flowcytometry and detected mitochondrial membrane potential change with DiOC_6(3) staining flowcytometry.We also used Annexin V-PE/MG double staining flocytometry to examine the mitochondrial changes in apoptosis progress.Results NAO staining results showed that 1 μmol·L~(-1) DEX stimulation reduced the cardiolipin content of thymocyte mitochondria(P<0.01).MG staining confirmed the mitochondrial mass decline with DEX treatment (P<0.01),and also found DEX reduced the DiOC_6(3) staining efficiency in thymocytes(P<0.01).Annexin V-PE/MG staining results showed that cell population with low mitochondrial mass existed in apoptotic thymocytes,and they were more in DEX group than in control group(P<0.01).Conclusion Mitochondrial mass declined in DEX induced mouse thymocyte apoptosis;mitochondrial mass evaluation is promising for the application to the apoptosis related immunological pharmacy study.