CS (chitosan) has emerged as a promising non-viral vector for gene delivery because of its ability to form complexes with pDNA (plasmid DNA) and enhance its transport across cellular membranes through endocytosis. Complexes of CS and pDNA may improve transfection efficiency; however, they are not capable of sustained DNA release and prolonging gene transfer. In order to achieve prolonged delivery of CS-DNA complexes, we prepared CS NP (nanoparticle) and CS-DNA complexes. alpha-Methoxy-omega-succinimidylpoly(ethylene glycol) was then conjugated to the surface of CS-DNA complexes using an active ester scheme; finally, the potential of PEGylation [poly(ethylene glycol)ylation] of CS NP as a non-viral gene-delivery vector to transfer exogenous genes in vitro and in vivo were examined. Electrophoretic analysis suggested that CS NPs could protect the DNA from nuclease degradation. The pDNA carried by CS NPs could enter and be expressed in HepG2 cells. However, the transfection efficiency was very low and the highest dose of DNA transferred was 1.6 microg. The transfection activities of CS-DNA-PEG were preserved and a higher dose (2.4 microg) of pDNA was transferred. This indicated that the transfection efficiency of the PEGylated complexes had been improved. In vivo experiments also showed that CS-DNA-PEG complexes mediated higher gene expression in tissues than did CS-DNA complexes, and that gene expression in tumours induced by CS-DNA-PEG complexes was the highest of all. These results suggested that PEGylation of CS-DNA complexes improves non-viral gene delivery in vitro or in vivo and has the potential to deliver therapeutic genes directly into hepatoma tissues.