Since globalization and offshoring trend have been widely adopted in supply chain, procurement and purchasing activities through the capable foreign supplier has been recognized as enablers to firms to achieve higher firm performance. Therefore, Supplier Development Program (SDP) is more and more important in buyer-supplier relationship. SDP not only contribute to the supplier to gain capabilities and higher performance, but also enables buyers to develop competitive advantages in the market. However, there still requires further investigation regarding what factors motivate the buyers to be involved in supplier development process. Furthermore, most of existing study focused on western markets and companies and their supplier development activities. To shed light on the literature gaps, this study adopted social capital theory as theoretical lens and investigate how relational factors that perceived by the buyer can impact them to involve in transaction-specific investment as an effort for supplier development, and how such investment can promote them to have superior market competitiveness. The study used 362 Korean based buying firm samples that have foreign suppliers in China, and analyse the data by using structural equation modelling technique. The study found development of social capital in the relationship promote the buyers to invest in the physical/human resources to the supplier and this in turn, contribute them to achieve higher market competitiveness. The results emphasized that development of relationship (level of cognitive, structural, relational capital) is enabler to firms to share transaction specific resources and thus quality of relationship is essential for the buyer-supplier relationship.