This paper proposes the complementarity and equivalence of core competencies between firms, as well as social capital, as factors determining strategic alliances. Specifically, the higher the complementarity of core competencies held by two firms, the more similar the magnitude of their core competencies, and the greater the number of past alliances between them, the higher the likelihood of alliance formation between the two firms. To test these hypotheses, this study used syndication data for new stock issuance underwriting among the top 98 securities firms in the United States during the 1980s. The empirical analysis results showed that not only the complementarity of tangible and intangible assets between firms, but also the equivalence in the magnitude of resources held, and social capital formed through past inter-firm cooperative relationships, increased the likelihood of alliance formation. These results demonstrate that the economic phenomenon of strategic alliances is influenced not only by economic logic but also by social logic. The findings of this study suggest that when firms enter into or dissolve strategic alliances with other firms, they should take a long-term perspective and carefully consider the formation and utilization of social capital in order to forge alliances with ideal partners in the future.