Several methods and tools have been developed to support managing project portfolios. However, despite the increasing attention on project portfolio management (PPM) over the last decade from both academic and practitioners, very few methods and tools have been proposed for modeling dependencies among projects within a portfolio. For better visualizing interdependencies among projects, this paper proposes to apply interpretive structural modeling (ISM) for structuring project interdependencies into a simple graphical hierarchical model. For demonstration, this method is applied to a portfolio consisting of 22 interdependent projects.