Colorectal cancer (CRC), the third most frequent occurred cancer, is associated with high mortality and extremely poor prognosis. Ginsenoside Rg3 (Rg3), one of the pharmacologically active components of traditional Chinese herbal medicine Panax ginseng, exerts antitumor effects against several types of cancer growth, including colorectal cancer. However, the detailed molecular mechanisms and particularly the signaling pathways that are decisive in this process are not yet fully elucidated. The present study was carried out to determine the antitumor effects of Rg3 using human colorectal cells in vitro and Xenograft tumor model of human colon cancer in vivo. We found that Rg3 effectively suppressed the proliferation of cancer cells in three human colorectal cancer cell lines (HCT116, HT29, SW480). In addition, intraperitoneal injection of Rg3 for 3 weeks significantly inhibited the growth of xenografts in nude mice. Furthermore, we determined the potential underlying mechanisms for these actions. Treatment with Rg3 significantly inhibited the transactivation of C/EBPβ and NF-κB, as well as the association of C/EBPβ with p65-NFκB in nucleus. However, when SW-480 cells were co-transfected with C/EBPβ, or pretreatment with TNFα, Rg3 failed to inhibit tumor growth. Taken together, our results revealed a robust anti-tumor effect of Rg3, which is mediated by inhibition of C/EBPβ/NF-κB signaling.