Background and Objectives: Nitric oxide (NO) is generated in mammalian tissue by the conversion of L-arginine to L-citrulline. This reaction is catalyzed by nitric oxide synthase (NOS). NO is an important bioactive agent and a signalling molecule that mediates a variety of biologic actions such as vasodilation, neurotransmission, host defense, and iron metabolism but increased NO production may also contribute to the pathogenesis of a various of disorders, including cancer. Before now, the role of NO in thyroid gland is still investigated and it was supposed that NO mediate the angiogenesis in tumor growth. Others journal and works identified the expression of iNOS that involve by neutrophil and eNOS that involve in part in the vascular remodeling and to understand the role of NO in human thyroid gland. But authors revealed only eNOS in thyroid neoplasm. iNOS was identifed by inflammation in fault. Materials and Methods: Western blot analysis was performed, using a polyclonal antibody against eNOS (Rabbit polyclonal IgG). Using the same antibody, the distribution of eNOS was examined in 15 formalin-fixed paraffin embedded samples by immunohistochemistry. By NADPH consumption rate, NOS activity was estimated at nodular hyperplasia. Results: Western blot analysis exhibited that eNOS was significantly elevated in thyroid papillary carcinoma, compared to that in nodular hyperplasia and normal tissue. Immunohistochemistry showed that the immunoreacitivity was present more significantly in thyroid follicular epithelial cell layer than vascular endothelial cell. NOS activity increased in nodular hyperplasia. Conclusions: Thyroid papillary cancer without neutrophil invasion expressed only eNOS. The endothelial localization of eNOS may play an important role in pathogenensis of human thyroid nodular hyperplasia and the follicular localization of thyroid papillary carcinomas.