Objective：To observe thyroperoxidase (TPO) expression in fine needle aspirated thyroid tumor cells and to assess the value of TPO immunostaining in differential diagnosis of benign and malignant thyroid tumors. Methods: Fine needle aspiration (FNA) samples and frozen tissue sections of 71 surgically resected thyroid tumor specimens were immunostained by with hTPO MoAb47. The staining results were considered positive if 80% or more thyroid cells were positively stained. Results: TPO expression levels in FNA samples and frozen tissue sections of malignant tumors were lower than those of benign tumors (P<0.01), and there was no difference in the results of FNA and frozen tissue sections. We also found that the TPO staining intensity of malignant tumors was markedly lower than that of benign tumors, with significant differences found between the mean densities, relative optic densities, and the intensities of staining color (all P<0.05), but with no significant difference found between the results of FNA samples and frozen tissue sections. TPO staining was positive in 20 of the 22 smears of the benign cystoma and in none of the 4 smears of malignant cystoid lesion (P<0.05). Taking the results of H-E staining as the control, the sensitivity of FNA samples was 95.74% and the specificity was 91.67%; of frozen tissue section were 100% and 66.47%; and of the cystoid lesions smears were 97.44% and 88.46%, respectively. Conclusion: TPO expression level is obviously higher in benign thyroid tumors than those in the malignant ones. FNA samples can be used for TPO immunocytochemistry staining as an adjuvant tool for distinguishing benign and malignant tumors.