It has been documented that over-expression of metastasis-associated in colon cancer-1 (MACC1) is related to poor prognosis in non-small cell lung cancer (NSCLC). This study investigates the function and underlying molecular mechanisms of MACC1 in lung adenocarcinoma. Here, we firstly employed immunohistochemistry, western blotting, real-time PCR, and online database to demonstrate that MACC1 expression was elevated in tumor tissues compared with tumor-adjacent or normal tissues. Real-time PCR, CCK-8, colony formation western blotting, Hoechst staining, and flow cytometry assays then evaluated the effects of MACC1 knockdown on the cell cycle, cell proliferation and apoptosis in A549 and H1299 adenocarcinoma cells. Result highlighted that MACC1 knockdown inhibited cell proliferation, induced G0/ G1 phase arrest and promoted cell apoptosis in vitro. Mechanistic analysis revealed it also up-regulated expression levels of bax, cleaved-caspase-3 and cleaved-PARP while down-regulating cyclin Dl, c-myc, bel-2, and beta-catenin expression in A549 cells. Intriguingly, up-regulation of beta-catenin suppressed G0/G1 phase arrest and apoptosis in MACC1-silenced A549 cells and this was accompanied by increased levels of cyclin Dl, c-myc, and bcl-2. Collectively, our results indicate that MACC1 knockdown effectively inhibited cell proliferation and promoted apoptosis of lung adenocarcinoma cells by regulating the beta-catenin pathway.