Vitamin D levels have been reported to be associated with diabetes, obesity and metabolic syndrome. There have been studies on the nutritional status of vitamin D in postmenopausal women at Seoul and premenopausal women at Busan, and these studies showed that nearly no relationship between serum vitamin D levels and the obesity index existed. However, there have been no studies that examined about the relationship between serum vitamin D levels and insulin resistance in Korea. In this study, we investigated serum vitamin D levels and the relationship between serum vitamin D levels and insulin resistance (homeostasis model assessment of insulin resistance), obesity index (body mass index, percentage of body fat and waist circumference) in 180 premenopausal women (non-obese women 87.8%, obese women 12.2%) in spring (March~April), fall (September~October) and winter (January~February) at Daejeon. Serum vitamin D levels were lower in winter than in spring-fall, after adjusting for age and the obesity index. The frequency of vitamin D inadequacy (serum vitamin D levels were $leq$ 20 ng/mL) was 45.5% in winter and, 23.5% in spring-fall, and which showed that vitamin D inadequacy was higher in winter than in spring-fall. Multiple regression analysis showed that serum vitamin D levels had no relationship with the obesity index or insulin resistance. There was no difference in the obesity index or insulin resistance between the vitamin D inadequacy and normal group, and there was no relationship between serum vitamin D levels and the obesity index or insulin resistance in non-obese and obese premenopausal women, respectively. In conclusion, serum vitamin D levels in premenopausal women at Daejeon were lower in winter than in spring-fall, and the frequency of vitamin D inadequacy was higher in winter than in spring-fall. Serum vitamin D levels had no relationship with the obesity index or insulin resistance in premenopausal women, most of whom were not obese.