This study investigates leisure activity, involvement and self-concept in cyberspace of various age groups of adolescents. A total of 1,388 students (elementary school=337, middle school=326, high school=361, university=364, consisting of 696 male and 692 female) participated in the study. The results are as follows. First, although the vast majority of adolescents (96.9%) had a computer at home, majority of adolescents visited Internet cafe. Second, 70.3% of adolescents visited Internet cafe to play Internet games, followed by engaging in information search, chatting, and participated in cyber community. Third, on average adolescents spent 5.43 hours per week playing Internet games, with more males playing Internet games than female adolescents. As for information search, the weekly average was 2.60 hours, with university students spending more time than the other groups. As for chatting, the weekly average was 1.69 hours, with no significant differences among the groups. The weekly average of Internet use was 9.65 hours, with older groups spending more time. The weekly average use of computer was 10.91 hours, with older groups spending more time and more males using more computer than females. Fourth, as for self-concept in cyberspace, elementary and middle school students reported that they had fun, while high school and university students reported that they were the same as in the regular daily life. In addition, adolescents reported that they spent leisure activity in cyberspace and they become a fictional character in cyberspace. Fifth, when they played Internet games, regardless of age and gender, adolescents reported that they had fun, followed by that they were absorbed, that they became aggressive, and that they were the same as in the regular daily life. Sixth, when they chatted on Internet, regardless of age and gender, adolescents reported that they had fun, followed by that they were not interested, that they were the same as in the regular daily life, and that they do not chat on Internet. Seventh, when they interacted with their friends on Internet, regardless of age and gender, majority of adolescents reported that they had fun, followed by that they conversed, that they were the same as in the regular daily life, and that they felt closer. These results indicate that Korean adolescents view Internet as a place to spend their leisure time and that they enjoyed spending time on Internet.