Objective To study the relationship between the expression of p16 protein and the biological behavour of thyroid tumor. Methods The expression of p16 protein was examined in fifty cases of thyroid tumor by immunohistochemical staining technique(SABC method). The relationship between p16 protein expression and tumor size, pathological type, differention grading and clinical stage of thyroid tumors were analysed. Results In thyroid adenoma, p16 protein expression was detected in 13 out of 15 cases(86.7%); in thyroid carcinoma p16 protein was expressed only in 16 out of 35 cases(45.7%), the difference was significant(P