This paper analyzes the effect of major diversity of club members on the start-up club effectiveness by focusing on the mediating effect of the conflict among the club members. Results of empirical analysis using data from 1,012 start-up club members are as follows. First, it has been shown that the major diversity of the start-up club members has a positive influence on conflicts among club members. These results are consistent with previous studies emphasizing the negative effects of diversity. When educational backgrounds of the club members are varies, the efficiency of decision-making and communication among the members is deteriorated and the coherence of the clubs is hindered, which may lead to conflicts among the club members. Second, the major diversity of club members has a negative effect on the effectiveness of start-up club. In other words, these results show that when the educational backgrounds of club members are diverse, members with different major backgrounds may have difficulty in developing complementary knowledge, which can have a negative impact on organizational effectiveness. Third, conflicts among club members have a negative effect on club effectiveness. These results are consistent with previous studies that focused on the negative aspects of conflict. In other words, conflict can interfere with organizational effectiveness of start-up club by causing tensions among members and breaking relations among the club members. Finally, the major diversity of the members showed a positive effect on the conflict, but a significant negative effect on the club effectiveness. Even when considering the conflict as a mediator, major diversity still has a significant negative effect on club effectiveness. Thus, conflict could be confirmed to be playing a role of a partial mediator in the relationship between the major diversity of the startup club members and the effectiveness of the startup club.