OBJECTIVE: The aim of the study was to evaluate NADH dehydrogenase [ubiquinone] iron-sulfur protein 8 (NDUFS8) serum concentration as a marker of Complex I, and the relationship with insulin resistance in type 1 diabetes mellitus (T1DM). DESIGN AND METHODS: Participants were adults with T1DM, recruited over the course of 1 year (2018-2019). NDUFS8 protein serum concentration was measured using the ELISA test. Insulin resistance was evaluated with indirect marker estimated glucose disposal rate (eGDR). The group was divided on the base of median value of eGDR (higher eGDR-better insulin sensitivity). RESULTS: The study group consists of 12 women and 24 men. Medians of eGDR and NDUFS8 protein concentration are 7.6 (5.58-8.99) mg/kg/min and 2.25 (0.72-3.81) ng/mL, respectively. The group with higher insulin sensitivity has higher NDUFS8 protein serum concentration, lower waist to hip ratio (WHR), body mass index (BMI), and they are younger. A negative correlation is observed between NDUFS8 protein serum concentration and WHR (rs = -0.35, = 0.03), whereas a positive correlation is observed between NDUFS8 protein serum concentration and eGDR (rs = 0.43, = 0.008). Univariate logistic regression shows a significant association between insulin sensitivity and lower age, as well as a higher NDUFS8 serum level. A multivariate logistic regression model confirms the significance (AOR 2.38 (1.04-5.48). = 0.042). Multivariate linear regression confirms a significant association between insulin sensitivity and better mitochondrial function (beta = 0.54, = 0.003), independent of age, duration of diabetes, and smoking. CONCLUSIONS: Higher NDUFS8 protein serum concentration is associated with higher insulin sensitivity among adults with T1DM.