Background and Objectives: The aim of this study was to investigate the relationship between major dietary patterns and the risk of insulin resistance (IR) among an urban Iranian population. Methods and Study design: In this longitudinal study, 802 adult men and women were studied within the framework of Tehran Lipid and Glucose Study. Fasting serum insulin and glucose were measured at baseline and again after a 3-year of followup. The usual dietary intakes were assessed using a validated 168 item semi-quantitative food frequency questionnaire and major dietary patterns were obtained using principal component analysis. Logistic regression models were used to estimate the occurrence of IR across tertiles of dietary patterns with adjustment for potential confounding variables. Results: Mean age of participants was 39.0±11.2 years and 45.5% were men. Three major dietary patterns including the Western, traditional and healthy were extracted, which explained 25.3% of total variance in food intake. The healthy dietary pattern, loaded heavily on intake of vegetable oils, fresh and dried fruits, low-fat dairy, nuts and seeds, was accompanied with a reduced risk of insulin resistance by 51% (OR=0.49, 95% CI=0.30-0.81), and 81% (OR=0.19, 95% CI=0.10-0.36), in the second and third tertile, respectively (p trend=0.001). In the presence of all dietary pattern scores in the logistic regression model, a 45% reduced risk of IR was observed per 1 unit increase in healthy dietary pattern score. Conclusion: These findings confirmed the protective effect of a plant-based, low-fat dietary pattern against the development of insulin resistance as a main risk factor of type 2 diabetes and metabolic disorders.