Specific antibody responses induced by inactivated H7N9 influenza vaccine were determined in BALB/c mice within a long period of time after immunization. BALB/c mice were intramuscularly injected once with the vaccine alone or plus MF59 adjuvant. Sera samples were consecutively collected from tail vein within 15 months following vaccination. Vaccinespecific IgG antibody titers were detected by ELISA. HI and neutralization antibody levels were also measured in samples of the last month. It was found that specific antibody levels increased slowly along with time and peaked during 5 months, then declined a little. However, they were relatively stable since then. The antibody levels were positively correlated with the vaccine dosage and were improved by addition of MF59. HI and MN tests showed that antibodies produced after 15 months of immunization could effectively neutralize virus and the neutralizing antibody titers were proportional to the amount of antigen. The experiments demonstrated that immunization once with H7N9 inactivated vaccine could induce relatively stable levels of specific antibodies and provide longterm protection for mice. Increase of vaccine dosage and addition of MF59 adjuvant could raise the humoral immune responses.