Abstract: Fifteen healthy piglets aged 28 to 30 day-old were divided into three groups at random(5 piglets per group), and were inoculated intramuscularly with mPCV2-C(ORF3 gene deleted mutational strain of PCV2), PCV2-CD(parent strain) and physiologic saline, respectively. By using the methods of PCR-RFLP and PCR, the DNA of mPCV2-C and PCV2-CD were found in the inguinal lymph nodes of slaughtered piglets from group A and B respectively, which proved that the mPCV2-C could replicate and proliferate in piglet body. Observation on histopathology showed that some pathological changes were found in the inguinal lymph nodes, liver cells and alveolar wall epithelium in slaughtered piglets of group A and B, in which that of group B was more serious, the results proved that the ORF3 gene deletion didn’t change the tissue tropism of PCV2. The dynamic determination of T lymphocyte subgroups in peripheral blood showed that, compared with group C, the percent of CD3⁺ and CD4⁺ T lymphocyte subgroups of group A and B decreased, and the CD8+ cell of group A returned high above that values of group B and C, which indicated that ORF3 gene deletion weakened the influences of PCV2 on T lymphocyte subgroups, and mPCV2-C demonstrated the tendency to induce cellular immunologic response in piglets. The results mentioned above also indicated the decrease tendency of pathogenicity of the virus with ORF3 gene deletion. The dynamic determination of antibody against PCV2 proved that mPCV2-C induced the humoral immunity in piglets obviously, and had good immunogenicity. mPCV2-C is a candidate strain for gene deletion vaccine against PCV2.