Thin-film hybrids of hydroxyapatite (HAP)/chitosan form in a supersaturated solution of HAP in the presence of poly(acrylic acid) (PAA). The cooperative interactions between chitosan, PAA, and calcium ion on the crystallization of calcium phosphate have been examined. In the presence of PAA, the crystallization of calcium phosphate occurs in the chitosan matrix, resulting in the formation of the hybrid thin films. Observation of the films by electron diffraction and X-ray diffraction confirm the deposition of hydroxyapatite.