Abstract: Abstract Aim： To study the effects of molecule weight，the N/P ratio，solvent and ionic strength on the formation and surface properties of PEI/DNA complexes，and study its transfection efficiency. Methods： The N/P ratio of the prepared PEI/DNA complexes was optimized using gel electrophoresis and UV quantitative assay.The particle size and surface charge of the complexes were measured in different solvents and ionic strength.The transfection efficiency in HepG2 cells was observed. Results： There was a positive correlation between combination of PEI and DNA and molecule weight of PEI.In addition，the surface properties of PEI/DNA complexes were also influenced by the solvents and ionic strength.Comparable transfection efficiencies in HepG2 cells were observed for the Lipofectamine 2000/DNA and the prepared PEI（25 kD）/DNA complexes in PBS at the N/P ratio of 12-15，which was much higher than that of naked DNA. Conclusion： The optimized PEI/DNA complexes could effeciently transfect the cells in comparison to the positive control.