Hydrodynamics-based administration via tail vein was used to deliver naked plasmid with human factor Ⅸ (hFⅨ) cDNA in 2.2 mL Ringer's solution into mice within 7 s. The peak level of expression of hFⅨ was 2921 ng/mL in mouse plasma. The hFⅨ cDNA expression increased with increasing the amount of plasmid DNA injected. The peak level of gene expression declined after repeated injection of plasmid (1459 ng/mL). The hFⅨ cDNA was detected in various organs, but the highest level of gene expression appeared in liver. Transaminase levels and liver histological results showed that rapid intravenous plasmid injection into mice induced transient focal acute liver damage, which was rapidly repaired within 3-10 d. These results suggested that high-level expression of hFⅨ cDNA can be achieved by hydrodynamics-based plasmid transfer and this method is now further used for gene therapy and gene function study in our lab.