Complex formation of poly(ethylene glycol)-poly(L-lysine) (PEG-PLL) AB type block copolymer with salmon testes DNA or Col E1 plasmid DNA in aqueous milieu was studied. The PLL segment of PEG-PLL interacts with nucleic acid through an electrostatic force to form a water-soluble complex associate with a diameter of ca. 50 nm. PEG segments surrounding the core of the polyion complex prevented the complex from precipitation even under stoichiometric conditions, at which the unit ratio of L-lysine in PEG-PLL and phosphate in the DNA is equal. The profile of the thermal melting curve revealed a higher stabilization of DNA structure in PEG-PLL/DNA complexes compared to that in the complex made from DNA and PLL homopolymer with the same molecular weight as the PLL segment in PEG-PLL. This stabilizing effect on the DNA structure may be due to the compartmentalization of DNA into the microenvironment of PEG with low permittivity. The reversible nature of the PEG-PLL/DNA complex was further verified through the addition of polyanion [poly-(L-aspartic acid)]: Poly(L-aspartic acid) replaced DNA in the complex with PEG-PLL, resulting in the release of free DNA in the medium. Furthermore, the PEG-PLL/DNA complex showed high resistance against DNase I attack, suggesting DNA protection through the segregation into the core of the associate having PEG palisade.