The importance of the human side of project management to assess the success of international development project has not been fully considered yet. An analysis of the literature on the project success definition, focused on the success criteria and success factors, was carried out. The organization’s effectiveness, in terms of Relations Sustainability, emerged as a criteria integrating the “time, cost, performance” approach to define a project success. Based on previous research contributions on the factors influencing the organization’s effectiveness, the paper expands the analysis of the influence of Organizational Climate on the Relation Sustainability between project manager and project team involved in international cooperation for development. The statistical methods used include confirmatory factors analysis and structural equation modeling. The results carry implications for project management identifying five dimensions of Organizational Climate (trust, innovation, social cohesion, communication and job challenge) influencing Relations Sustainability. This finding suggests that Organizational Climate contributes to project success by creating trust, stimulating commitment and generating satisfaction to overcome conflicts between project manager and project team