Objective: To explore the expressions of Nuclear Kappa B (NF-κB) and matrix metalloproteinase-9 (MMP-9) in placental tissues of pregnant rats with subclinical hypothyroidism and study the occurrence mechanism of threatened abortion. Methods: The expressions of NF-κB and MMP-9 in placental tissues of pregnant rats with subclinical hypothyroidism and normal pregnant rats were detected by immunohistochemistry and Western blotting assay. Correlations between the expressions and the occurrence mechanism of threatened abortion of subclinical hypothyroidism were analyzed. Results: The positive rates of NF-κB and MMP-9 in placental tissues of pregnant rats with subclinical hypothyroidism were 16.7% and 50% ,respectively,while in normal placental tissues were 83.3% and 100% ,respectively.The differences were significant(all P<0.05).The results of Western blotting assay were consistent with those of immunohistochemistry. Conclusion: The expression levels of NF-κB and MMP-9 in placental tissues of pregnant rats with subclinical hypothyroidism have a high correlation with the invasion，and joint detection of the above indicators ，and it is instructional for judgement the occurrence mechanism of threatened abortion of subclinical hypothyroidism.