Business networks are emerging as an indispensable form of organization in the contemporary production system. The social capital theory provides an original grid (Nahapiet and Ghoshal, 1998) to understand their development. Our objectives are to analyze the building of social capital to identify factors in the development of business networks. The research involves the study of a network of thirty French SME’s specialized in the metalwork sector. At a time when governments are questioning the conditions for the success of approved business networks our results offer further insight into their building and more specifically on the emergence and mobilization of social capital.