BACKGROUND: Lung adenocarcinoma is the most common pathological type of lung cancer. However, the mechanisms underlying its development are still poorly understood. B7-H3 was discovered as a new member of the B7 costimulatory family. METHODS: We detected the expression status of B7-H3 protein in lung adenocarcinoma tissues, and evaluated the relationship of B7-H3 expression and patients' prognosis. Then, we silenced its expression in A549 cells by transient siRNA transfection to ascertain the function of B7-H3 in lung adenocarcinoma cells. Western blotting was used to detect the expression of epithelial-mesenchymal transition (EMT) related proteins. RESULTS: We found that B7-H3 overexpressed in lung adenocarcinoma. It is correlated with lymph node metastasis, distant metastasis, and disease stage. The Cox regression analysis showed that B7-H3 might serve as an independent prognostic marker of lung adenocarcinoma. We also found that B7-H3 promoted proliferation, invasion and migration of A549 cells in vitro. B7-H3 also could promote EMT progression by regulating EMT-related molecules. CONCLUSION: B7-H3 is a potential target for the treatment of lung adenocarcinoma.