Nano-structured hydroxyapatite(HAp)/chitosan composites and their transparent films were prepared by the co-precipitation and solvent casting method. According to TEM observation, HAp particles with 100~150nm in length and 10~20nm in width were formed in the composites. We investigated the role of citric acid in the HAp/chitosan film formation. The HAp/chitosan films were prepared at various concentrations of citric acid. HAp particles did not aggregate in the chitosan matrix above 50％ citric acid concentration for the HAp content. The inhibition effects result from the adsorption of citric acid on the surface of HAp particles by ionic exchange of phosphate ions with citrate ions at the HAp/chitosan composite solution, which is caused by a higher affinity of citrate than phosphate species for the Ca-site on the HAp surface.