Type I diabetes mellitus is the most common metabolic disorder in childhood and adolescence that as an autoimmune disorder is significantly associated with other autoimmune diseases. The aim of this study was to determine the prevalence of thyroid autoimmunity and thyroid autoimmune disease in patients with type I diabetes. In this cross-sectional study 119 patients with type I diabetes were enrolled. Profile of patients included age, sex, duration of diabetes, age at onset of diabetes, medications, history of different disease and body mass index were evaluated and recorded. Thyroid examination was carried out by endocrinologist. Body weight was measured without shoes using a digital scale and height also was measured in standing situation using a stadiometer. All blood samples were collected between 8 and 9 am and after 8 hours fast. Fasting plasma glucose (FPG), HbA1C, free thyroxin (fT4), thyroid stimulating hormone (TSH), Anti-thyroid peroxidase (anti-TPO), Anti-thyroglobulin (anti-Tg) was measured. Quantitative variables were expressed as mean (and standard deviation) and qualitative variables as a percentage. All the analysis was done by software STATA version 12. In this study, a total of 119 patients with type I diabetes were attended that positive anti-TPO was found in 34 cases (28.6%) and positive anti- Tg was also found in 15 cases (12.6%). All the subjects who had positive anti-Tg at the same time had positive anti- TPO. Overt hypothyroidism was observed in 12 cases (10.1%) and subclinical hypothyroidism in 18 patients (15.1%). Eightynine subjects (74.8%) also had normal thyroid function. Thyroid autoimmune disorders in patients with type I diabetes has high prevalence. Hypothyroidism is more than other types of thyroid disorders. Due to the high incidence of thyroid dysfunction in diabetic patients with positive antibody titers, periodic evaluation of thyroid function in these patients appears to be essential for early diagnosis and treatment.