Objective: Double (unstable) diabetes is generally defined as type 1 diabetes mellitus (TIDM) with insulin resistance (IR). This combination can lead to inadequate glycemic control even with higher insulin doses, thereby probably increasing risk of cardiovascular (CV) complications. The phenotype of double diabetes may be related to the genetic and lifestyle factors, and weight gain caused by intensive insulin therapy. The aim of this study was to investigate the clinical and metabolic parameters according to the degree of IR in Korean TIDM. Methods: We evaluated 203 patients with TIDM (men 100, women 103) whose fasting C-peptides were ≤ 0.6 ng/mL and who were treated with insulin. Insulin sensitivity was assessed by the rate constant of plasma glucose disappearance (kitt%/min) using short insulin tolerance test in all patients,: the low tertile of kitt was considered relatively insulin-resistant group. And visceral fat thickness (VFT) and carotid intima-media thickness (CIMT) were measured by ultrasonography. Results: In both genders, fasting glucose and HbA1c levels were significantly higher in the low tertile group than in the high tertile one. It should be clinically emphasized that the fluctuation in blood glucose (hyperglycemia and hypoglycemia) might be severe in spite of high insulin dose to maintain euglycemic state. Interestingly enough, there were no differences in VFT and CIMT, as well as DM duration, body mass index, waist circumference, and lipid profiles between the two groups. Conclusion: Except for difficulties related to glycemic control, the influence of IR on most of metabolic parameters in Korean double diabetes was not clear. However, this result suggests that lifestlye modification and the use of insulin sensitizers should be used to control unstable hyperglycemia in double diabetes.