We developed pDNA/chitosan complexes coated with sugar-modified PEG-A/Cs, which are poly(ethylene glycol) derivatives with a side chain of carboxylic acid and sugar. The cationic pDNA/chitosan complexes were coated with anionic sugar-modified PEG-Cs, and anionic ternary complexes were formed. Coating the pDNA/chitosan complexes with maltoseor lactose-modified PEG-A/C (Mal-PEG-A/C or Lac-PEG-A/C, respectively) greatly promoted their stability in water and transfection efficiency in vitro.