What factors facilitate efficient cooperation between alliance partner firms in joint ventures? How does inter-firm and inter-departmental cooperation evolve over time? This paper provides implications regarding the interrelationships among theories related to alliance strategy, learning, trust, and cooperation. Cooperation is influenced by the purpose of the alliance, ownership and management, task complexity, differences in organizational structures and procedures, and the presence or absence of trust. This study adopted qualitative research methods used in anthropology (participant observation, interviews, document analysis, etc.) and was based on approximately three years of field research, contract reviews, and interviews. Therefore, it sought to reveal the nature of causal relationships and processes through a deeper understanding of actual inter-firm and inter-departmental interactions. While ostensibly focusing on Renault Samsung as a single company, the study comparatively analyzed the process of cooperation development through successive new product development projects involving three companies: Samsung, Nissan, and Renault. First, complementary capabilities between IJV alliance partner firms are a necessary but not sufficient condition for efficient cooperation in the initial stage. In practice, fostering an atmosphere of reciprocity and mutual learning is important for maintaining long-term and effective cooperative relationships. Meanwhile, ownership and control rights directly influence inter-firm cooperative relationships as the joint venture comes to resemble the organizational structure and decision-making procedures of the parent company. However, as the cases of Samsung and Nissan demonstrate, ownership is not necessarily a prerequisite for building efficient cooperative relationships. Second, the formation of trust relationships is crucial for building effective cooperative relationships. Developing and establishing new cooperative processes between partner firms or departments is necessary because it serves as a foundation for building trust. Organization-wide commitment and learning play a very important role in building inter-firm trust relationships. Organization-wide cooperative commitment originates from interpersonal relationships and enables inter-departmental trust to take root. Inter-departmental trust relationships can persist as long as mutual learning and informal interpersonal relationships are maintained, even when trust between partner firms weakens. Third, when developing new products internally within the joint venture, despite the efficiency of timely technology transfer from the parent company, the effectiveness of technology transfer diminishes if the joint venture's organizational capabilities are not adequately developed. Therefore, receiving manageable new products with parental support is critically important for the joint venture's success. Similarity in organizational structures and decision-making processes between the parent company and the joint venture subsidiary positively influences mutual cooperation.