Background: Diabetes mellitus is a well-established risk factor for the development of coronary artery disease (CAD), which is the major cause of premature death in diabetic patients, both in type 1 or type 2 diabetes. Objective: To examine the relationship between insulin resistance determined by fasting insulin level and complexity of coronary artery lesion in diabetic patients. Patient and methods: This study enrolled 84 diabetic patients who were presented to the catheter Lab for elective coronary angiography in the period from October 2019 to April 2020. All patients were subjected to full history taking, thorough clinical examination, laboratory investigations (fasting insulin level, HbA1c level, lipid profile and serum creatinine), 12-lead surface ECG, echocardiography examination and coronary angiography. The complexity of the coronary artery lesions was evaluated using Gensini score. Statistical tests were used to analyze the association between the measured fasting insulin levels and Gensini score. Results:there was a statistically highly significant positive linear correlation between insulin level and Gensini score (P<0.05). The accuracy of for insulin level as a predictor for coronary artery disease equals 96,6% by 96.7% sensitivity and 75% specificity at cut off predictive value <25.8 mIU/L. Conclusion: Fasting insulin level is a simple test for insulin resistance and significantly correlated with severe CAD so it may have a value as a predictor of severity of CAD and risk stratification of diabetic patients.