The existing literature on inter-partner characteristic differences and alliance performance in global strategic alliances has not moved far beyond seemingly contradictory conventional wisdoms such as "complementary differences between partners are essential for success" or "combinations of similar partners yield better performance." What kinds of differences or diversity between partners actually improve the performance of strategic alliances? What kinds of similarities lead to better performance? In such cases, are there ways to mitigate the negative effects that may arise from differences? Despite its importance, comprehensive and systematic research on inter-partner characteristic differences and alliance performance that can answer these questions has been difficult to find. This study identifies the novel concepts of "competing similarity" and "cooperating similarity" as dimensions of inter-partner similarity/diversity and comprehensively analyzes their effects on alliance performance. Additionally, "relational efforts" between partners, which mitigate the effects of cooperating similarity, were introduced as a moderating variable. Data collected through a mail survey generally supported this study's hypotheses that competing similarity has a negative effect on alliance performance while cooperating similarity has a positive effect. Furthermore, the hypothesis that mutual efforts to enhance inter-partner relationships moderate the effect of cooperating similarity on alliance performance was also partially supported.