[Abstract] Objective: To investigate the correlation between PD-L1 expression and dMMR related proteins in follicular thyroid carcinoma tissues and its clinical significance. Methods: The postoperative paraffin-embedded tissue samples from 60 patients with thyroid follicular carcinoma were collected from the Second Affiliated Hospital of Fujian Medical University during January 2015 and June 2020. The collected samples were re-confirmed as thyroid follicular carcinoma tissues by Hematoxylin-eosin staining. The expression of PD-L1 and four homologous proteins encoded by four genes (MLH1, MSH2, MSH6, PMS2) in MMR system were detected by immunohistochemistry in the cancer and paracancerous tissues. The relationship between the expression of PD-L1 and depletion of MMR related proteins in thyroid follicular carcinoma tissues and its clinical significance were analyzed. Results: The positive expression rate of PD-L1 was significantly higher in the follicular thyroid carcinoma tissues than that in paracancerous tissues [63.3%(38/60) vs 11.7%(7/60), P<0.05]. The expression of PD-L1 was significantly correlated with tumor diameter, extrathyroidal infiltration, vascular invasion and recurrence (all P<0.05). In the cancer tissue specimens from 60 patients, 24 (40.0%) had expression of four MMR related proteins, which were pMMR tumors, and 36 (60.0%) had depletion of one or more MMR related proteins, which were dMMR tumors. The dMMR-type thyroid follicular carcinoma was significantly correlated with the status of lymph node metastasis and tumor staging (all P<0.05). PD-L1 was positively correlated with the incidence of dMMR, and PD-L1 was an independent risk factor for disease recurrence, while dMMR was associated with a better prognosis. Patients with PD-L1+/pMMR type were associated with higher tumor malignancy, while patients with PD-L1+/dMMR type were not associated with tumor pathological features but may easily benefit from immunotherapy. Conclusion: Positive PD-L1 expression and dMMR highly occur in follicular thyroid carcinoma. PD-L1 is associated with the increased tumor invasion and is an independent risk factor for disease recurrence, while dMMR is an early molecular event in the development of thyroid follicular carcinoma and is associated with better prognosis of patients.