Calcium ions were implanted into a titanium surface in quantities of 10¹⁶, 10¹⁷, and 10¹⁸ ions/cm². The ion-implanted surface was characterized using surface analysis techniques to determine the structure of the surface-modified layer formed by the implantation. The specimens were immersed in a neutral electrolyte solution whose composition resembled that of extracellular fluid. After immersion, the surface of the specimen was again characterized. The data revealed that the surface oxide of titanium grew after the calcium-ion-implantation. The surface-modified layer of the calcium-ion-implanted titanium was composed of titanium dioxide, which contained a calcium oxide and a complex oxide of calcium and titanium near the surface. The precipitation of calcium phosphate on titanium in electrolyte is tremendously accelerated by calcium-ion implantation, which is a surprising property for a biomaterial.