The third complementarity-determining region (CDR3) of TCR interacts directly with antigenic peptides bound to grooves of MHC molecules. Thus, it is the most critical TCR structure in launching acquired immunity and in determining fates of developing thymocytes. Since length is one of the components defining the CDR3 heterogeneity, the CDR3 length repertoires have been studied in various T cell subsets from humans in physiological and pathological conditions. However, how the CDR3 length repertoire develops has been addressed only by a few reports, including one showing that CDR3 of CD4 thymocytes becomes shorter during thymic development. Here, we explored multiple regulations on the development of the TCRB CDR3 length repertoires in the thymus and the peripheral blood. CDR3 length spectratyping was employed to examine thymocyte and peripheral T cell populations for their CDR3 length repertoires. We have found that repertoire distribution patterns depend on use of the BV gene. The BV-dependent patterns were shaped during thymic selections and maintained in the peripheral blood. Differences in the mean CDR3 length among different BV subsets were seen throughout lymphocyte development. We also observed that CDR3 was shortened in both CD4 and CD8 thymocytes. Of note, the degrees of the shortening depended on the CD4/CD8 lineage and on use of the BV gene. When expansions of peripheral T cell clones are negligible, no obvious difference was seen between mature thymocytes and peripheral lymphocytes. Thus, the TCRB CDR3 length repertoires are finely tuned in the thymus before the lymphocytes emigrate into the peripheral blood.