The effects of DNA conformation and carrier DNA on the efficiency of electroporation are studied using the expression of erythropoietin cDNA in CHO cells as an indicator. It turns out that 250mg/L carrier DNA increases the expression level by 3 fold,and linearized plasmid DNA is more suitable than supercoiled DNA to obtain stable transfection by electroporation. The results suggest that it is essential to linearize plasmid DNA and cotransfect with apporporiate carrier DNA to obtain stable and high level expression of foreign genes by means of electroporation.