Ginseng has been used as a medicinal herb to maintain physical vitality for thousands of years, and it has also been shown to be a nonorgan-specific cancer preventive agent by several epidemiologic studies. However, the chemopreventive effects of Korea white ginseng (KWG) in lung squamous cell carcinoma (SCC) have not been tested. In this study, we investigated the chemopreventive activity of KWG in a mouse lung SCC model. N-nitroso-trischloroethylurea (NTCU) was used to induce lung tumors in female Swiss mice, and KWG was given orally. KWG significantly reduced the percentage of lung SCCs from 26.5% in the control group to 9.1% in the KWG group and in the meantime, increased the percentage of normal bronchial and hyperplasia. KWG was also found to greatly reduce squamous cell lung tumor area from an average of 9.4% in control group to 1.5% in the KWG group. Treatment with KWG decreased Ki-67 staining, suggesting that the lung tumor inhibitory effects of KWG were partly through inhibition of proliferation. High-performance liquid chromatography/mass spectrometry identified 10 ginsenosides from KWG extracts, Rb1 and Rd being the most abundant as detected in mouse blood and lung tissue. The tumor inhibitory effects of KWG are mediated by inhibition of activator protein (AP-1), as showed by in vitro study conducted on AP-1/NF-κB-dependent mouse non-small cell lung carcinoma cell lines. Western blotting of lung tissues also indicated that NTCU upregulated AP-1 through phosphorylation of c-jun-NH2-kinase, which was downregulated by KWG in concurrence with its chemoprevention function. These results suggest that KWG could be a potential chemopreventive agent for lung SCC.