We have previously shown that synthetic octacalcium phosphate (OCP) displays highly osteoconductive and biodegradable characteristics. However, OCP cannot be sintered without thermal decomposition due to the existence of water molecules in the structure. The acquisition of the moldability and the improvement of the handling property in this material are subjects for the clinical use. In the present study, we prepared OCP complex with hyaluronic acid (Hya) that could be used in the injectable form and further examined the bone tissue reaction to cortical bone by placing the complex directly on an 8-weeks-old ICR mouse calvaria in comparison with the placement of OCP granules only. The granule form of OCP (between 300 to 500 μm in diameter) was mixed with sodium hyaluronic acid with molecular weights 90 x 104. The complex revealed an injectable characteristic if it was utilized in a syringe. After polytetrafluoroethylen ring was mounted on mouse calvaria, the inner space of the ring was filled with the complex and left the complex as it is for 6 weeks. Histological examination using the decalcified specimens indicated that the OCP/Hya complex exhibited greater bone formation than OCP granules only group within the ring at 6 weeks. The results suggested that the OCP/Hya complex could be used as an injectable and osteoconductive bone substitute material in many clinical situations.