Granzyme B (GzmB) and perforin are proteins, secreted mainly by natural killer cells and cytotoxic T lymphocytes that are largely responsible for the induction of apoptosis in target cells. Because type 1 diabetes results from the selective destruction of β cells and perforin deficiency effectively reduces diabetes in non-obese diabetic (NOD) mice, it can be deduced that β cell apoptosis involves the GzmB/perforin pathway. However, the relevance of GzmB remains totally unknown in non-obese diabetic (NOD) mice. In this study we have focused on GzmB and examined the consequence of GzmB deficiency in NOD mice. We found that NOD.GzmB^–/– mice developed diabetes spontaneously with kinetics similar to those of wild-type NOD (wt-NOD) mice. Adoptive transfer study with regulatory T cell (T_reg)-depleted splenocytes (SPCs) into NOD-SCID mice or in-vivo T_reg depletion by anti-CD25 antibody at 4 weeks of age comparably induced the rapid progression of diabetes in the NOD.GzmB^–/– mice and wt-NOD mice. Expression of GzmA and Fas was enhanced in the islets from pre-diabetic NOD.GzmB^–/– mice. In contrast to spontaneous diabetes, GzmB deficiency suppressed the development of cyclophosphamide-promoted diabetes in male NOD mice. Cyclophosphamide treatment led to a significantly lower percentage of apoptotic CD4⁺, CD8⁺ and CD4⁺CD25⁺ T cells in SPCs from NOD.GzmB^–/– mice than those from wt-NOD mice. In conclusion, GzmB, in contrast to perforin, is not essentially involved in the effector mechanisms for β cell destruction in NOD mice.