A viral marketing has been an effective marketing tool in various fields. In this study, we applied a viral marketing strategy for managing the Internet community website of scientists and engineers, KOSEN(www.kosen21.org). We developed a function that enables members to send promotional e-mails to their acquaintances, and we also designed a reward system for the members who participated in KOSEN viral marketing. With these strategies, KOSEN members reacted positively on the rewards (i.e., mileage points, gift card) and also the loyalty members took part in the viral marketing program more actively than other members. Furthermore, an attractive event was very effective for the prompt reaction of the viral marketing. Managing loyalty members is necessary for the continuous effects. The viral marketing strategies and analysis results of this research could be used for the management of an Internet service.