This paper is to prove empirically that the cooperative business structure and conflict control capability of a platform leader and that the complementarity of a platform will have positive impacts on whether complementors select specific platforms in 245 platform selection examples of 99 application developers. Past research has not been sufficient regarding platform selection, but has been similar to other research on ERP platforms. Therefore, it is very meaningful to make an empirical study on the platform selection of complementors, which has not yet been performed. As a result, it is important for platform leaders to establish their own cooperative business structures and secure the complementarity of their platforms. Therefore, platform leaders should create more transaction opportunities by helping complementors to develop more complements and making business ecosystems grow more and more.