Environment factors, including maternal or infant dietary nutrition have been reported to have an influence on the pathogenesis of type 1 diabetes. In the present study, to investigate the effect of maternal or post-weaning offspring's nutrition, in particular the essential fatty acid ratio (n-6/n-3) on the development of type 1 diabetes, we prepared two kinds of chows with n-6/n-3 ratios of 3.0 (L) and 14.5 (H), and provided them to mothers of non-obese diabetic (NOD) mice during gestation and lactation and to the offspring after weaning. The n-6/n-3 ratios in breast milk and erythrocyte membrane of NOD offspring became nearly the same with that of the maternal diet at 2 weeks after birth. In the L chow-fed offspring from L chow-fed mother (LLL), levels of insulitis were higher than those in the H chow-fed offspring from H chow-fed mother (HHH) at 4 weeks of age, while the levels in the LLL offspring became lower than those in the HHH after 6 weeks. Early insulin autoantibody expressions were found from 2 to 6 weeks in the HHH offspring, but not in the LLL. The LLL offspring exhibited strong suppression of overt diabetes development in regard to the onset and accumulated incidence of diabetes compared to the HHH. The study with combined L and H chows during gestation, lactation in mother and in post-weaning offspring revealed that only the LLH chow significantly suppressed the development of diabetes with similar kinetics to LLL chow, although the other combinations may delay the onset of diabetes. The present findings suggest that n-6/n-3 ratio of the maternal diet during gestation and lactation rather than that of offspring after weaning strongly affects the development of overt diabetes in NOD mice.