Hydroxyapatite/chitosan composites are being researched as useful biomaterialsfor bone repair and regeneration. In order to make a biomimetic composite, the in vitro formationof apatite on the surface of the acetylated chitosan film using a simulated bodyfluid was tested. The formation and in-situ crystallization behavior of hydroxyapatite as afunction of the degree of acetylation was investigated. The surface of the acetylated chitosanwas densely covered by forming a cluster of a plate-shaped crystalline phase with athickness of approximately 20 nm, and the crystalline cluster was identified with apatite,including hydroxyapatite. The closer the acetylation rate of chitosan was to 50%, the largerwere the amount of crystalline phases formed, and the Ca/P ratio gradually increased toreach 1.67 of hydroxyapatite. As the ratio of glucosamine and acetylglucosamine in chitosanbecame identical, the formation of apatite was maximized and hydroxyapatiteaccounted for most of them.