Calcium phosphate-polymer composites have been produced for bone-repairing. We have focused development of composites by a crystal growth technique in a hydrogel matrix, that is regarded as gel-mediated processing. Under the gel-mediated condition, reaction temperature is one of the major parameter to determine microstructure of the precipitated crystals. In the present study, we investigated effects of the reaction temperature on formation of calcium phosphates through gel-mediated processing where double diffusion technique was applied. Crystalline phases of calcium phosphate formed in the hydrogel were varied from octacalcium phosphate (OCP) to hydroxyapatite (HAp) with increasing the reaction temperature. OCP crystals formed at 4 and 40 °C had granular or spherical shape, while HAp crystals formed at 80 °C had rod shape. The HAp crystals were composed of a large number of fibrous crystals. The rod-shaped HAp crystals were oriented in the direction of ion diffusion. Formation of oriented HAp crystals was generated by increase in diffusion rate of ions in the hydrogel matrix after increasing reaction temperature in gel-mediated processing.