Abstract: Objective To investigate the expression of miRNA-26a in breast tissues and cells as well as its impact on the proliferation and invasion of human breast cancer cells. Methods Real time polymerase chain reaction (RT-PCR) and Western blot were employed to detect the expression of miR-26a and COX-2 in 20 cases of breast cancer tissues and corresponding para-carcinoma tissues, human breast cancer cells (MCF-7 and BT474) and human breast cells MCF-10A. After breast cancer cells MCF-7 and BT474 were respectively transfected with miR-NC and miR-26a, Western blot was employed to detect the COX-2 expression in transfected cells. CCK-8 and clone formation experiments were employed to determine the proliferation and invasion ability of transfected breast cancer cells. Results The expression of miR-26a in breast cancer tissues was significantly lower than that in para-carcinoma tissues (t=20.33, P=0.001). The expression of miR-26a in MCF-7 and BT474 cells were significantly lower than those in MCF- 10A cells(Dunnett t test I-J=-0.031, P=0.001). In contrast, the COX-2 expression in breast cancer tissues and cells were significantly higher than those in para-carcinoma tissues (t=18.01, P=0.002) and human breast cells (Dunnett t test I-J=-0.028, P=0.000), respectively. The expression of COX-2 was significantly decreased in miR-26a transfected cells. CCK-8 and clone formation experiment results revealed that the proliferation (F=6.032, P=0.013) and invasion (Dunnett t test I-J=-0.21, P=0.037) abilities of breast cancer cells MCF-7 and BT474 were markedly inhibited by the overexpression of miR-26a. Conclusion The overexpression of miR-26a could remarkably down-regulate COX-2 expression, to inhibit the proliferation and invasion abilities of human breast cancer cells.