Polyethylenimine (PEI) has been used as cationic polymers for efficient gene transfer without the need for endosomolytic agents. Various kinds of PEIs with different molecular weight were tested in order to investigate the effects of the molecular weight of PEI on the transfection efficiency and cell cytotoxicity. The ${eta}-galactosidase$ expression $(pCMV-{eta}-gal)$ plasmid was used as a model DNA. Complex formation between PEI and pDNA was assessed by 1% agarose gel electrophoresis method. Particle size and zeta-potential of complexes were determined by electrophoretic light scattering spectrometer. In vitro transfection efficiency was assayed by measuring ${eta}-galactosidase$ activity. Cell cytotoxicity was determined by MTT assay. Particle sizes of the complexes became smaller on increasing molecular weights of PEI and N/P ratios. Surface potential of complexes was increased as the molecular weight of PEI increased. Transfection efficiency of $pCMV-{eta}-ga1$ on the HEK 293 cells was greatest with PEI 25 K system but having the lowest cell viability. PEI with high molecular weight showed higher transfection efficiency and cell viability than PEI with low molecular weight.