Hydroxyapatite (HAp) is widely used as bioceramics for bone and dental tissue reconstructions due to its excellent biocompatibility with hard tissues and high osteoconductivity. Although it is well known that HAp has the high adsorbed ability, its ability is decreased in high ionic solution. To immobilize protein on the HAp surface, this study demonstrates that Collagen (Col) and fibronectin (Fnt) were immobilized on the surface of hydroxyapatite (HAp) sintered body with and without the silane coupling agent of aminopropyltriethoxysilane (APS). The proteins immobilized on the HAp and APS/HAp surfaces were investigated by atomic force microprobe (AFM, Simadzu; SPM 9500) analyses and Ζ potential measurements. The stability of protein/APS/HAp and protein/HAp composites was evaluated after immersion in phosphate buffer and NaCl solutions with various concentrations. AFM analyses and Ζ potential measurements revealed that proteins immobilized on the APS/HAp are more stable than those immobilized on the HAp in high ionic solutions.