Objective To investigate the changes of peripheral blood and immune cell phenotypes between agedand young C57BL/6J mice. Methods Peripheral blood cell count, immune organ coefficient and peripheral blood immunecell phenotype in aged C57BL/6J mice (19 months) and 2-month-old C57BL/6J mice used as control adult mice wereanalyzed. Results Compared with young mice, the aged mice had an increased peripheral blood platelet count, increasedproportion of neutrophils, and a decreased proportion of lymphocytes ( P <0. 001). The thymus coefficient was significantlyreduced ( P <0. 001). The percentage of CD4+ cells in the peripheral blood immune cell typing was obviously reduced ( P <0. 001). Conclusions The blood cells in aged mice are differentially skewed, and helper T-cell numbers declined. Thechanges are similar to those in the aging human. This study provides basic data for research on immuno-senescence.