This study explains the performance of Korean Systems Integration (SI) firms based on the characteristics of ecological niches identified through network analysis. To this end, the structure of ecological niches—a concept frequently used in organizational ecology and strategic management theory but rarely measured systematically—was assessed through network analysis of joint involvement networks among firms competing in the same business domains, and the effects of the structural characteristics of each firm's ecological niche on performance were analyzed. Specifically, this study assumed that competitive ties exist among firms participating in the same business domain and, through network analysis, examined four ecological niche characteristics: betweenness centrality, eigenvector centrality, network efficiency, and network constraint. Hypotheses were tested regarding whether these four independent variables each affect net income, which served as the performance variable. The results of hypothesis testing revealed that three of the four variables—betweenness centrality, eigenvector centrality, and network efficiency—had statistically significant effects in the directions predicted by the hypotheses.