Background: Complications of diabetes mellitus could be decreased by maintaining a good glycemic control, which is dependent on adherence to self–care and lifestyle modification Objectives: To assess the non–adherence to lifestyle modifications and self–care management and its determinants among diabetic mellitus patients in Thiruvallur district. Methods: This was a cross sectional study with 500 diabetic patients were selected by simple random sampling method from June 2017 to October 2017 using pre–tested semi–structured questionnaire. Results: 21.4% were aged above 60 years and 79% were females. Non adherence rate for lifestyle modifications and self–care management are diabetic diet plan (81%), physical activity (68.8%), blood glucose monitoring (20.2%), foot care (69.6%) and for medication (1.80%). Conclusion: Most common reason for non–adherence was lack of knowledge and this supports the need for intensive education and awareness about self–care and life style modifications of diabetes mellitus.