To overcome the critical barriers in gene delivery, a series of reducible polycations (RPCs) based on low molecular weight (LMW) peptides, i.e. PolyHK6H, PolyHK6H-mPEG1, PolyHK6H-mPEG2, and PolyHK6H-mPEG3, with different poly(ethylene glycol) (PEG) contents, are synthesized and evaluated as nonviral gene vectors. All the RPCs exhibit lower cytotoxicity compared with 25 kDa polyethyleneimine (PEI) and PEGylated PEI (PEI-mPEG: PEI-mPEG1, PEI-mPEG2, and PEI-mPEG3). PolyHK6H-mPEG1 and PolyHK6H-mPEG2 can bind and condense plasmid deoxyribonucleic acid (pDNA) efficiently with a particle size of about 200 nm. Moreover, they display much higher transfection efficiency than that of 25 kDa PEI especially in serum-supplemented medium. Moreover, PolyHK6H-mPEG1 has equal transfection efficiency with PEI-mPEG1 which is optimal in the PEI-mPEG, but PolyHK6H-mPEG1 exhibits significantly lower cytotoxicity than PEI-mPEG1. This is attributed to the fact that inter-peptide disulfide bonds can increase the stability of RPCs/pDNA complexes in extracellular environment and thereafter cleave in cytoplasm to facilitate the release of pDNA in intracellular environment. The PEGylated RPCs demonstrate here improved intracellular gene transfer performance and will have great potential applications in vivo.