In this paper herein, we have conducted a series of case studies concerning a number of methods on R&D cooperation between large-sized firms and small & medium-sized firms for a measure to recognize the importance of South Korea's small and medium-sized firms and reinforce their R&D capability. In particular, in-depth analyses were conducted on the success factors and complementary factors in R&D capability between large-sized firms and small & medium-sized firms by analyzing cases of new product development with conditional purchase option that can be a signature case of R&D capability between large sized-firms and small & medium sized-firms in South Korea. In this paper, we also conducted questionnaire analysis and interviews. The R&D cooperation between large-sized firms and small & medium-sized firms includes all kinds of cooperations between large-sized firms and small and medium-sized firms among industry- academies-research. The interview for this study was conducted and focused on the 3 following types: public institutions and small & medium-sized firms, university and small & medium-sized firms, and large-sized firms and small & medium-sized firms. We identify some policy implications, First, we suggest the policy measures that can create synergy through cooperation by analyzing merits and demerits of large-sized and small & medium-sized firms. Second, we suggest some measures that can enhance the cooperation performance of large-sized and small & medium-sized firms and improve the cooperation relationships between them. Third, we suggest additional policy measures that can effectively promote R&D cooperation, CEOs' intentions, common goal awareness, outside network application, designated personnel placement, geological adjacency, and information protection management.