Endochondral ossification is essential for new bone formation and remodeling during the distraction stage. Endochondral ossification is attributed to chondrocyte maturation, which is induced by various factors, such as the cellular environment, gene transcription, and growth factor expression. Cartilage oligomeric matrix protein (COMP)-angiopoietin 1 (Ang1) is more soluble, stable, and potent than endogenous Ang1, and COMP-Ang1 treatment has osteogenic and angiogenic effects in an in vivo model of bone fracture healing. Although the osteogenic effects of COMP-Ang1 have been demonstrated, the precise mechanism by which COMP-Ang1 induces chondrocyte maturation and triggers endochondral ossification is not understood. Here, we investigated the possible mechanism by which COMP-Ang1 induces chondrocyte maturation. First, using a WST assay, we found that COMP-Ang1 is nontoxic in rat chondrocytes. Then, we isolated total RNA from COMP-Ang1–treated rat chondrocytes, and analyzed the decrease in chondrogenic gene expression and the increase in osteogenic gene expression using real-time RT-PCR. Gene and protein expression of heme oxygenase-1 (HO-1), which maintains chondrocytes in an immature stage, decreased in a dose-dependent manner upon COMP-Ang1 treatment. To clarify the relationship between HO-1 and COMP-Ang1 in chondrocyte maturation, we used cobalt protoporphyrin IX (CoPP IX), an HO-1 inducer, and tin protoporphyrin IX (SnPP-IX), an HO-1 inhibitor. Treatment with various combinations of CoPP IX, SnPP IX, and COMP-Ang1 confirmed that COMP-Ang1 accelerates chondrocyte maturation by reducing HO-1. In conclusion, our results suggest that COMP-Ang1 accelerates chondrocyte maturation by interacting with HO-1. J. Cell. Biochem. 114: 2513–2521, 2013. © 2013 Wiley Periodicals, Inc.