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 effi-ciency 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 Lipo-fectamine 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.