The purpose of the present study was to examine the effect of vitamin K₂ on cancellous and cortical bone mass in rats with streptozotocin (STZ)-induced type 1 diabetes. Twenty-seven male Sprague-Dawley rats aged 12 weeks were randomized by the weight-stratified method into the following three groups: age-matched control group, STZ + vehicle group, and STZ + vitamin K₂ group. STZ (40 + 50 mg/kg) was administered intravenously twice during the initial 1-week period. Vitamin K₂ (menatetrenone, 30 mg/kg) was administered orally 5 days a week. After 12 weeks of treatment, the serum glucose concentration and femoral length and weight were measured and histomorphometric analysis was performed on the cancellous and cortical bone of the distal femoral metaphysis and femoral diaphysis, respectively. STZ administration induced hyperglycemia and a decrease in femoral weight. The STZ + vehicle group also showed cancellous osteopenia due to a decrease in the number of osteoblasts/bone surface (N.Ob/BS) and the osteoblast surface (ObS)/BS without any significant changes in bone-resorption parameters, but it did not have a significant decrease in cortical bone mass. Administration of vitamin K₂ to STZ-treated rats prevented the development of hyperglycemia and a decrease in femoral weight. Vitamin K₂ also prevented cancellous osteopenia by inhibiting the decrease in N.Ob/BS and ObS/BS without significantly affecting bone-resorption parameters, but it did not significantly increase cortical bone mass. These results suggest that vitamin K₂ has beneficial effects on glucose concentration and cancellous bone mass in rats with STZ-induced type 1 diabetes.