Fluorescence correlation spectroscopy (FCS) and gel electrophoresis measurements are performed to investigate both the number and size of complexes of linear double-stranded DNA (dsDNA) fragments with 1: 1 diblock copolymers consisting of a cationic moiety, branched polyethyleneimine (bPEI) of 2, 10, or 25 kDa, covalently bound to a neutral shielding moiety, poly(ethylene glycol) (PEG; 20 kDa). By systematically decreasing the bPEI length, the PEG grafting density along the DNA chain can be directly controlled. For 25 and 10 kDa bPEI-PEG copolymers, severe aggregation is observed despite the presence of the shielding PEG. Upon decreasing the bPEI length to 2 kDa, controlled self-assembly of monomolecular DNA nanoparticles is observed. The resulting complexes are in quantitative agreement with a theoretical model based on a single DNA encased in a dense PEG polymer brush layer. The resulting PEGylated complexes show high stability against both salt and protein and hence are of potential use for in vivo gene delivery studies.