For the determination of glutathione S-transferase activity, a new method was established by using HPLC system. Moreover, amount of enzyme for a optimum reaction was determined by a comparative study with a variety concentration of enzyme. Using a established method, activity of glutathione S-transferase that is alcohol metabolizing enzyme was investigated on the fruit of Hovenia dulcis Thunb. As the result of experiment, EtOH and $H_2O$ extracts of the fruit of Hovenia dulcis Thunb showed visible a synergistic effect of glutathione S-transferase activity. On a continuous experiment, EtOH and $H_2O$ extracts of the fruit of Hovenia dulcis Thunb showed alcohol decomposition activity on the in vivo test using rat. These results suggest that the fruit of Hovenia dulcis Thunb may be useful in the prevention of hangover.