Protein adsorption on hydroxyapatite particles with different morphologies was investigated. The nano-sized hydroxyapatite (nano-HAp) was prepared using calcium nitrate tetrahydrate [Ca(NO₃)₄·4H₂O] and diammonium hydrogen phosphate [(NH₄)₂HPO₄] by using a hydrothermal method with varying synthesis temperature and pH conditions. The adsorption properties of proteins onto nano-HAp were studied using three types of proteins: bovine serum albumin (BSA, an acidic protein), myoglobin (MGB, a neutral protein), and lysozyme (LSZ, a basic protein). The adsorption amount of BSA and LSZ per unit specific surface area of nano-HAp increased as the crystallinity improved. In contrast, the improvement in the crystallinity of HAp decreased adsorption affinity to MGB. This adsorption behavior on nano-HAp would be considered by the specific binding of the C- or P-site on HAp towards acidic or basic proteins, respectively.