Chitosan-coated hydroxyapatite (CS-HAp) nanoparticles have been prepared successfully by a modified dropping method. Variations of the final solution pH values, concentration of CS, and CS/HAp volume ratio were examined for their effects on particle size, intensity of surface charge, and tendency of particle formation. The smallest nanoparticles carry a positive charge with 40-50 nm in width and 60-70 nm can be obtained when the CS/HAp ratio is 1 by controlling the final solution pH value at 6.5. The ability of nanoparticles to complex DNA, toxicity of nanoparticles, and the effect of CS-HAp nanoparticle on transferring DNA were also investigated. Transgene expression in HeLa and NIH3T3 cell on the nanoparticles were significant after 2 days. Agarose gel electrophoresis indicated a probable multiple interaction between DNA and CS-HAp while the interaction between DNA appears to be only ionic interaction. No toxic effect on the cells was seen with CS-HAp. Lipofectin, however, was observed to engender marked cytotoxicity. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121: 3531-3540, 2011