Objective:To explore the expression of microRNA-21 (miR-21) in papillary thyroid carcinoma (PTC) and its effect on proliferation and apoptosis of K1 cell. Methods:Real-time quantitative PCR(qRT-PCR) was performed to detect and compare the expressions of miR-21 in 12 pairs of PTC tissues and adjacent normal tissues. K1 cell was respectively transfected with Anti-miR-21 and mimic-miR-21 and their corresponding negative control groups were constructed. The expression of miR-21 in K1 cells was identified by qRT - PCR. Cell proliferation after transfection with anti-miR-21 was analyzed using MTT assay. The cell apoptosis of K1 after transfection was analyzed by flow cytometry. Western blotting was performed to detect the expressions of proliferation and apoptosis-related proteins. Results:Compared with adjacent normal tissues,miR-21 in PTC tissues was significantly up-regulated (P < 0.05). Compared with the negative control (Anti-miR-NC) group,the miR-21 expression of K1 cells was significantly down-regulated after Anti-miR-21 transfection(P < 0.01). MTT assay showed that the inhibition of miR-21 expression significantly reduced the proliferation of K1 cells(P < 0.05). Flow cytometry showed that K1 apoptosis rate after Anti-miR-21 transfection was significantly increased to 19.5%,while the negative control (Anti-miR-NC) group was 9.4%. Western blotting showed that the expression of Bcl-2 was down-regulated and the expression of Bax was up-regulated in the anti-miR-21 treated K1 cells. Conclusion:The expression of miR-21 is up-regulated in PTC. Down-regulated expression of miR-21 significantly inhibits proliferation while promotes the apoptosis of K1 cell.