Recent studies have revealed that the maturation state of vessels in tumors, in addition to vascularity, is a critical determinant of tumor growth. The role of oxygen-dependent signaling pathways in hypoxia-stimulated angiogenesis is well established, however, little is known about their impact on vessel maturation in tumors. Here, we have studied the function of the cellular oxygen sensor, factor inhibiting HIF-1 (FIH), which controls the activity of hypoxia-inducible factor-1. FIH silencing in mouse LM8 osteosarcoma stimulated angiogenesis but did not influence tumor growth. In contrast, FIH overexpression led to increased pericyte coverage of the tumor vasculature, reduced vessel leakiness and enhanced tumor growth. Vessel maturation was paralleled by up-regulation of platelet-derived growth factor (PDGF)-C in tumors and expression of PDGF receptor-α on pericytes. Ablation of PDGF-C in FIH-overexpressing tumor cells reduced pericyte coverage and tumor growth. Our data suggest that FIH-mediated PDGF-C induction in LM8 osteosarcoma stimulates the recruitment of PDGFR-α positive pericytes to the tumor vasculature, leading to vessel maturation and enhanced tumor growth.