Abstract: The immunogenicity of porcine circovirus type 2 (PCV2) Cap protein (rCap) expressed secretively by Pichia Pastoris and adjuvant effect of recombinant porcine interferon alpha (rPoIFNα) were examined in piglets in the present experiment. Groups of 30day old piglets which were negative of PCV2 antibody were inoculated with recombinant Cap protein in the presence or absence of rPoIFNα in a primeboost protocol. Then the piglets were challenged with PCV2 JS strain at 14 days post the boostvaccination. Neutralizing antibody against PCV2 was produced in piglets inoculated with Cap protein. The use of rPoIFNα resulted in significantly higher ELISA antibody and neutralizing antibody levels than recombinant Cap protein inoculation. After challenge, all control piglets developed viremia and had a long time fever, only one of four piglets receiving recombinant Cap protein of PCV2 developed viremia, while addition of rPoIFNα produced an improved protection to the viral challenge, since PCV2 replication was completely inhibited. As evaluated by viremia ratio, clinical signs (fever), growth parameters, the piglets were protected against PCV2 challenge after vaccination of rCap. rPoIFNα showed potential adjuvant effects on recombinant Cap protein of PCV2, which was characterized not only in antigen specific humoral immune response, but also in protection against PCV2 infection.