Abstract: Objective Thyroid adenocarcinoma is one of the most common malignancies in the endocrine system,while thyroid adenoma is the most common thyroid benign tumor.Some research has showed the difference in the gene expression between benign and malignant thyroid tumor.The aim of this study was to investigate the expression and clinical significance of biological rhythm gene CLOCK in thyroid carcinoma and adenoma. Methods Total 74 cases of thyroid carcinoma and 55 cases of thyroid adenoma were collected from March,2012 to March,2015 in our hospital.The expression of CLOCK was detected by immunohistochemical method.We analyzed its relationship with clinicopathological features. Results The positive rates of CLOCK in thyroid carcinoma and thyroid adenoma were 68.9%(51/74) and 32.7%(18/55),respectively,the difference was significant(P<0.001).The expression of CLOCK in thyroid carcinoma was 4.2±3.8,and in thyroid adenoma was 2.1±2.0,there was significant difference(P<0.001).The positive expression rate of CLOCK in cancer tissues with lymph node metastasis was significantly higher than that without lymph node metastasis(P=0.012).The positive expression rate of CLOCK was significantly correlated with the tumor size(P=0.028),and the CLOCK positive expression rate increased with TNM stage(P=0.023). Conclusion Up-regulation of CLOCK is not only involved in the formation of thyroid carcinoma,but also plays an important role in the process of tumor invasion and metastasis.