Ring-opening reaction of low molecular weight polyethylenimine with an Mw of 800 Da (800 Da PEI) with methylthiirane produced thiolated polyethylenimine (PEI-SHX ). The thiolation degree X, which is the average number of thiol groups on a PEI molecule, was readily adjusted by the methylthiirane/PEI ratio. Oxidation of the thiolated PEIs with DMSO afforded disulfide cross-linked PEIs (PEI-SSX ). The molecular weights of PEI-SS X were estimated by viscosity measurement to be 7100, 8000, and 8400 for X=2.6, 6.5, and 9.4, respectively. The PEI-SSX series can bind and condense plasmid DNAs effectively forming nanosized polyplexes. The size of dry polyplexes is less than 100 nm on the TEM pictures. In solution, the size of the polyplexes was measured by DLS to be about 400 nm. In vitro experiments showed that the PEI-SS X series have a lower cytotoxicity and higher gene transfection efficiency compared with the high molecular weight PEI with Mw of 25 KDa. The presence of fetal bovine serum did not decrease the transfection efficiency. The results proved the hypothesis that reductively degradable disulfide-containing PEIs could possesses simultaneously higher gene transfection efficiency and lower cytotoxicity than the nondegradable ones.