Introduction: Hashimoto's thyroiditis is autoimmune disease that effect on thyroid gland tissue via penetration some special white blood cells (called lymphocyte) into thyroid gland and it is act to product specific antibodies act to damage the thyroid cells. Vitamin D is fat soluble vitamin, it is from steroid compounds derivatives and act after binding with its receptor in target cell. Vitamin D has many functions, but the important function is regulation factor on gene level, so it include in regulation of autoimmune diseases severity.  Objective: This study aimed to explain the role vitamin D deficiency on severity of Hashimoto's thyroiditis via it's effect on TPO-Ab production. Method: The current study involved compare tow groups Hashimoto's thyroiditis with vitamin D deficiency as group 1 and Hashimoto’s thyroiditis without vitamin D deficiency group 2. The serum specimens of all individuals used to measure anti TPO-Ab, T4, TSH and vitamin D levels via TOSOH instrument. Results: This study result's explain after comparison between Hashimoto's thyroiditis with vitamin D deficiency group and Hashimoto's thyroiditis without vitamin D deficiency group the elevation of anti- TPO-Ab levels with sever hypothyroidism in Hashimoto's thyroiditis with vitamin D deficiency group more than other group.