Objective: Efficacy of insulin degludec/aspart(IDegAsp) compared to basal insulin in type 2 diabetes have not been studied extensively, with conflicting results. We investigated effect of changing basal insulin to IDegAsp and clinical factors related with the effects through multicenter studies. Methods: We retrospectively analyzed clinical data of adults with type 2 diabetes who visited endocrinology outpatient clinic in 3 referral hospitals. Those who had used basal insulin at least 4 months with HbA1c less than 11.0 % were enrolled. The basal insulin was replaced with IDegAsp from November 2017 to May 2019, and the clinical and laboratory data were collected at 2 and 6 months after the replacement. Results: A total of 80 patients were recruited (52.5% male, mean±SD of age 67.04 ± 9.80 years, duration of diabetes 18.87 ± 8.49 years, HbA1c 8.69 ± 0.97 %, C-peptide 1.63 ± 1.37 ng/mL, insulin dose 0.36 ± 0.14 IU/kg, eGFR 73.1 ± 31.3 mL/min/1.73㎡). HbA1c significantly decreased by changing to IDegAsp at 2 months (8.28 ± 1.10 %, P-value = 0.0001). In the meanwhile, the HbA1c at 4 months before the changing to IDegAsp had been 8.45 ±0.98%, which suggested aggravation of glycemia (P-value = 0.0080) during maintenance of basal insulin. Insulin dose continuously increased during the study period (P-value < 0.0001). These significant changes were also observed in subgroup analysis with 35 patients whose 6-month data were available. When we analyzed according to C-peptide levels and hemoglobin glycation index (HGI) at baseline, prominent improvements in HbA1c were found in the patients with low C-peptide (<0.7 ng/mL) and high HGI (≥ 1.4 %). Conclusion: We observed significant blood glucose lowering effect of IDegAsp in type 2 diabetic patients with poor glycemic control with basal insulin, especially the patients had severe insulin deficiency and glycemic variation.