I completed the 3 doses (0, 1, 6 months) of hbv engerix b at february 2005.yesterday, i found my ab level was <2iu/l, how should i retake the vaccine?

Yes: It is not clear why some persons seroconvert or make good antibody levels to a vaccine after the first or second dose and others require more vaccine exposures. While i'm not a specialist in hep b, it seems reasonable to re-vaccinate and repeat antibody testing after 3-4 weeks. It's also possible that a protective level occurred after 2005, but the antibody titer waned or decreased subsequently.