The surface structure and properties of synthetic colloidal calcium hydroxyapatite, Ca₁₀(PO₄)₆(OH)₂ (abbreviated as CaHap), were investigated. The CaHap surface possesses three kinds of P-OH groups acting as adsorption sites for H₂O, CO₂, CH₃I, CH₃OH and so on. The number of surface P-OH groups was ca. 2.6 groups nm⁻². When the CaHap was outgassed above 400^oC, the surface P-OH groups were dehydroxylated to form surface P-O-P groups. Based on the results obtained, the surface design of CaHap by modifying it with various inorganic and organic molecules was made. The hydrophilic CaHap surface turned into a hydrophobic surface by modification with monoalkyl phosphates and hexamethyldisilazane. The surface P-O-Si(CH₃)₃ groups of CaHap modified with hexamethyldisilazane were combusted at 500^oC in air to form surface P-O-Si(OH)₃ groups. The number and types of surface P-OH groups of the CaHap could be controlled by treating with pyrophosphoric acid.