Background: Type 1 diabetes mellitus (T1DM) is a common Pediatric autoimmune disorder. Children with type 1 diabetes mellitus are more prone to develop other organ-specific autoimmune diseases, among which autoimmune thyroiditis (AIT) is more frequently encountered. Objective: the present study designed to determine the seroprevalence and biochemical characteristics of autoimmune thyroiditis among children with insulin-dependent diabetes mellitus in Damietta Governorate in the period from October2011 to October2012. Patients and Methods: This cross sectional analytic study included 90 diabetic children receiving insulin therapy. They were selected from diabetic outpatients' clinic of Health Insurance Hospital in Damietta governorate; 54 of them were females and 36 of them were males. All patients were subjected to history taking, examination to detect major complications of diabetes & signs of other autoimmune disorders especially thyroiditis and laboratory investigations in the form of Hemoglobin A1C, Thyroid stimulating hormone (TSH), Anti-thyroid peroxidase antibody (TPO), Anti-thyroglobulin antibody (Tg) and Free tri-iodothyronine (T3)& Free thyroxin (T4) for patients with abnormal TSH. Thyroid ultrasound for all subjects with seropositive anti- thyroid autoantibody was also performed.Results: Mean age of studied cases was 11.2±3.3 years. Most of our cases were more than 6 years (87.8%), with duration of diabetes less than 3 years (62.2%), had positive family history of diabetes (70%), and negative family history of autoimmune diseases (66.7%). Prevalence of seropositive thyroiditis was 10 cases (11.1%) out of 90 patients, 8 (8.9%) of them shows picture of thyroiditis with neck ultrasound. Among patients with seropositive thyroiditis, there was no significant risk as regard sex of the patients (8 females and 2 males; P: 0.17), age (all patients more than 6 years; P: 0.2), and duration of diabetes (P: 0.59). There was significant difference as regard positive family history of autoimmune diseases (7 cases; P: 0.009). Regarding thyroid function status, 2.2% were suffering from hypothyroidism and were complaining only from goiter; TSH level was only increased in the two cases with low free T4 and low free T3. There was statistically significant difference as regard TPO Abs mean in negative cases and positive thyroiditis cases (2.74±8.03 and 263.69±231.77) respectively, and TG Abs mean in negative cases and positive thyroiditis (7.48±7.31 and 205.69±211.51). Conclusion: The presence of thyroid antibody positivity and the subsequent development of subclinical autoimmune thyroiditis were prevalent among the children with T1DM of our study. Positive family history of thyroid disease and abnormality in neck ultrasound were significant risk factors. Recommendations:All patients with T1DM should be screened for autoimmune thyroiditis upon diagnosis, and in case of thyroid antibody positivity they should be regularly followed up in terms of their thyroid function and growth status.