We investigated preventive effects of chronic endurance exercise on the slow-to-fast fiber transition in the plantaris muscles of non-obese type 2 diabetic rats. Eleven-week-old male Spontaneously Diabetic Torii Rats were used as diabetic animals, and assigned to sedentary (DB) and exercise (DB+Ex) groups. Age matched male non-diabetic Sprague-Dawley rats were used as sedentary control (Cnt) group. DB+Ex group was trained on low-intensity treadmill running for 14 weeks. Although glucose level in DB group at 25 weeks of age was higher compared to Cnt group, that in DB+Ex group was significantly lower compared to DB group. DB group exhibited lower percentages of high-oxidative type IIA fibers and higher percentages of low-oxidative type IIB fibers compared to Cnt group in the superficial layer of muscles. While DB+Ex group exhibited higher percentages of type IIA fibers and lower percentages of type IIB fibers compared to DB group in the superficial and deep layers of muscles. Additionally, succinate dehydrogenase intensity in the deep layer of muscles in DB+Ex group was increased by endurance exercise. The present study indicated that endurance exercise could prevent the slow-to-fast fiber transition of the muscle fibers and enhance mitochondrial oxidative enzyme activity in the skeletal muscles of non-obese type 2 diabetic rats.