Start-ups organizations are increasing rapidly. To overcome “liability of newness,” it is essential for entrepreneurs to compose the right co-founding team. Based on the review of 57 journal articles on start-up co-founding teams and new venture teams, we found that the composition of the team, especially diversity among team members, was frequently studied. However, previous research has only focused on surface-level diversity such as gender, age, functional background, previous experience. Only a very few studies investigated deep-level diversity such as co-founding team members’ value, personality or thinking style. The present study explored what type of diversity is required in start-ups. Since this topic is rarely studied, we first conducted a qualitative study by interviewing with nineteen start-up founders and venture capitalists in Study 1. As a result we found that four individual characteristic factors (extraversion, agreeing to different idea, risk taking, optimism) and four work-related factors (business opinion, speed oriented, big-picture oriented, time perspective) were the key component of deep-level diversity. In Study 2, we conducted a quantitative study to empirically investigate these aspects by a survey to thirty start-up related individuals. The result confirmed that the diversity were required for six aspects deducted from Study 1. For the other factors of optimism and speed orientation, frequency tendency supported Study 1’s result.