In order to identify whether 3ABC antibody positive swine inmmunized by vaccination were caused by virus infection or residual nonstructural protein（NS）in vaccine，detection of 3ABC antibody and virus infection in immunized animals was carried out，and the relationship between the appearance of 3ABC antibody and the dose of immunity was explored. Four groups（A，B，C and D）of 20 piglets at 60-day-old were injected with the Foot-and-mouth disease type O inactivated vaccine（OS/99 strain）at doses of 1，2，4 and 6 mL/ head，respectively. Sera and nasopharyngeal swabs were collected 1 days prior to immunization，and 3，7，14，30 and 60 days after immunization. NS antibodies and virus infection were detected by 3ABC indirect ELISA Kit，multiplex RT-PCR detection kit for FMDV，and virus infection detection. It was found that nonstructural protein antibodies were not detected in piglets after immunization at low doses (1 or 2 mL/heads). Whereas the positive rate of NS antibodiy was increased after immunization at high doses (4 or 6 mL/head)，and no virus infection or viral nucleic acid was detected in the animals with positive NS antibody. It was suggested that NS antibodies in the immunized swine were associated with vaccine doses.