The surface structure of Hydroxyapatite (HAp) particles during hydrothermal synthesis and their protein adsorption behavior was investigated. The HAp particles were prepared by mixing calcium acetate solution and diammonium hydrogen phosphate solution by hydrothermal synthesis. When the temperature of mixture were heated up to 120°C, 150°C and 180°C, the HAp particles were collected during hydrothermal synthesis. The adsorption properties of proteins onto HAp were studied using three types of proteins: bovine serum albumin (BSA), myoglobin (MGB) and lysozyme (LSZ). Surface analysis by BET revealed that their pore volumes were decreased by increasing synthesis temperature. The adsorbed amount of BSA and LSZ per unit milligrams showed no obvious difference in all of the HAp particles prepared with synthesis temperature. In contrast, the amount of MGB adsorbed onto HAp particles synthesized at 120°C, 150°, and 180°C shows the decreasing with an increasing synthesis temperature. This result suggests that the adsorbed amount of MGB was decreased by decreasing pore volume of HAp particles.