This thesis treats the supplier-customer relationship from a customer network perspective to examine the effect of supplying firm’s centrality in its downstream customer network on supplying firm’s CSR (Corporate Social Responsibility) performance. Furthermore, this research chooses supplier’s firm size as the moderator to examine the interaction effect of supplier’s centrality in customer network and supplier’s firm size on supplier’s CSR performance. Then, this study tests the mediating effect of supplying firm’s financial performance by the three-step method and Bootstrap method. I collect a data sample of 196 focal firms and 2520 supplier (focal company) -customer relationships in the computer and electronic product industry in 2019 to test the proposed hypotheses. The results show that (a) supplier’s centrality (degree/eigenvector centrality) in customer network positively affect supplier’s CSR performance; (b) supplier’s firm size will reduce the positive effects of supplier’s centrality in customer network on its CSR performance; (c) supplier’s financial performance will partially mediate the relationship between supplier’s centrality in customer network and its CSR performance. These findings combine the social network theory and resource-based theory and contribute to the understanding of both supply chain network and firms’ CSR performance.