The preparation and cell transfection of chitosan-DNA nanoparticles were studied. The TFPI (tissue factor pathway inhibitor) or EGFP (enhanced green fluorescent protein) plasmid DNA was encapsulated with chitosan to form gene nanoparticles. The results with TEM showed that the nanoparticles were of sphere shape. The mean diameter of the nanoparticles was 149 nm and the diameter ranged from 80-250 nm, which were measured by the photo related spectrometry (PCS). The encapsulation efficiency of DNA was 96% +/- 1.38% and the DNA content in the nanoparticles was 37% +/- 3.0%. The encapsulated DNA could be protected from the degradation by DNase I. The transfection efficiency of chitosan nanoparticles were about equivalent to that of the LipofectAMINETM reagent. Our results also showed that chitosan nanoparticles were nontoxic to cultured cells.