Endothelial progenitor cell (EPC) transplantation is a promising treatment option for ischemic diseases, and understanding the mechanism mediating angiogenic effects of EPCs is essential to further improve its effectiveness as well as their application to in vitro tissue engineering. Here we investigated EPC contribution on network formation using an in vitro three-dimensional network model. Bovine microvascular endothelial cells (ECs) were seeded on collagen gel to create a three-dimensional network model. Subsequently, EPCs isolated from rat bone marrow were seeded on top of a confluent endothelial cell (EC) monolayer to create EPC+EC model. In this model, EPCs promoted EC network formation. Quantitative analyses revealed that the total length, number, and depth of networks were significantly enhanced with the addition of EPCs. EPCs tended to localize in networks formed inside collagen gel rather than on a confluent monolayer with increasing experimental duration. In addition, EPCs preferentially distributed near sprout positions in a confluent monolayer. Furthermore, EPC-conditioned medium promoted network formation, and vascular endothelial growth factor was detected in the conditioned medium. Taken together, EPCs contributed to network formation by direct incorporation with on-site growth factor secretion. The angiogenic ability of EPCs offers a possible cell source to reconstruct vascularized tissues in vitro.