Functional foods targeting the ubiquitin-activating enzyme (E1) of the ubiquitin–proteasome pathway as anti-tumour agents are considered to be important for cancer chemoprevention. The low-toxicity Panax ginseng was used in traditional herbal medicine or food use for either treating or preventing cancer for more than 1000 years. In this study, we constructed an ubiquitin-tagged protein as a substrate to analyze the six ginsenosides Rb1, Rb2, Rc, Rd, Re, and Rg1 by ubiquitination assay. More ubiquitinated E1 levels were induced by increasing concentrations of ubiquitin-activating enzyme. Ginsenosides Re and Rg1 inhibited ubiquitin-activating enzyme. Unlike ginsenosides Re and Rg1, ginsenosides Rb1, Rb2, Rc, and Rd increased ubiquitination on E1 enzyme. Interestingly, ginsenoside Rg1 inhibits both the chymotrypsin-like activity of 26S proteasome and E1 activity. These results suggest that ginsenoside Rg1 is a potential functional food application for cancer prevention because of its specific E1 inhibitory effects. Furthermore, the 6-O-Glu position of the ginsenoside may play a role in its E1 inhibitory property. Ginsenosides could be further developed as a useful drug for cancer therapy.