This study investigated the characteristics of on-line auction users with addictive tendencies. A questionnaire survey was conducted on-line between September 9 and 14, 2005 and 1,608 adults who had experience in on-line auctions participated. As a result, 23.5 percent of the subjects in this study were shown to have addictive tendencies in their use of on-line auctions. The socio-demographic factor found was gender difference. That is, men were more likely to show addictive tendencies than women. The subjects who had addictive tendencies showed certain psychological characteristics such as lower self-esteem more symptoms of depression, and a higher sense of alienation compared to those without addictive tendencies. Finally, on-line auction users who had addictive tendencies were shown to visit on-line auction site more frequently and to stay logged on the auction site longer than non-addicts, reflecting that the frequent the use of on-line auction, the higher the possibility of addiction to on-line auction.