Forming coalitions and working collaboratively are important means of inter-organizational coordination. During coordination activities, organization’s evaluation of candidate collaborative projects and subsequent decisions on forming coalitions are often influenced by the ideas of their peers in the interorganizational network. Using humanitarian coordination as a case study, we propose a formal model for the emergence of coalitions in the context of inter-organizational networks. This model incorporates both the characteristics of orgnaizations and the inter-organizational network as factors for network influence. It has been implemented in an agent-based simulation for the study of humanitarian coordination.