A block catiomer polyplex, showing a high stability in the extracellular medium and an efficient release of plasmid DNA (pDNA) in the intracellular compartment, was developed by controlling both the cationic charge and disulfide cross-linking densities of the backbone polycations. Poly(ethylene glycol)-poly(L-lysine) block copolymer (PEG-PLL) was thiolated using either of two thiolation reagents, N-succinimidyl 3-(2-pyridyldithio)propionate (SPDP) or 2-iminothiolane (Traut's reagent), to investigate the effects of both the charge and disulfide cross-linking densities on the properties of the polyplexes. The introduction of thiol groups by SPDP proceeded through the formation of amide linkages to concomitantly decrease the cationic charge density of PLL segment, whereas Traut's reagent promoted the thiolation with the introduction of cationic imino groups to keep the charge density constant. These thiolated PEG-PLLs were complexed with pDNA to form the disulfide cross-linked block catiomer polyplexes, which had the size of approximately 100 nm. Both thiolation methods were similarly effective in introducing disulfide cross-links to prevent the polyplex from the dissociation through a counter polyanion exchange in the extracellular oxidative condition. On the other hand, the efficient release of pDNA responding to the reductive condition mimicking the intracellular environment was only achieved for the polyplex thiolated with SPDP, a system compensating for the decrease in the charge density with the disulfide cross-linking. This distinctive sensitivity toward oxidative and reductive environments was nicely correlated with the remarkable difference in the transfection efficiency between these two types of thiolated polyplexes (SPDP and Traut's reagent types): the former revealed approximately 50 times higher transfection efficiency toward 293T cells than the latter. Obviously, the balance between the densities of the cationic charge and disulfide cross-linking in the thiolated polyplex played a crucial role in the delivery and controlled release of entrapped pDNA into the microenvironment of intracellular compartment to achieve the high transfection efficiency.