Abstract: To study the influence of porcine circovirus type 2 (PCV2) infection on immune response of pig to classical swine fever (CSF) vaccine, twenty 28dayold healthy weaning piglets were randomly divided into four groups of five (Group VI, IV, V and C). The piglets in Group VI were intramuscularly injected with CSF vaccine two days before being inoculated with PCV2, piglets in Group IV were infected by PCV2 two days before being injected with CSF vaccine, piglets in Group V were only vaccinated by CSF vaccine and Group C as the blank control without inoculating anything. Piglets were vaccinated twice at an interval of twentyone days and blood samples were collected periodically so as to detect CSFVspecific serum antibody, lymphocyte proliferation activity and mRNA expression levels of IFNγ, IL2 and IL4 in peripheral blood lymphocyte (PBLC). PCV2 infection, either before or after vaccination, reduced CSFVspecific serum antibody production, seroconversion and lymphocyte proliferation activity. After the primary vaccination, expression of IFNγ, IL2, IL4 and IL10 mRNA decreased seriously in PBLC from piglets in Group VI and Group IV, especially in Group VI. Following the secondary vaccination, expressions of IFNγ and IL10 mRNA in PBLC from Group VI and Group IV appeared seriously unbalanced and those of IL2 as well as IL4 mRNA were deficient, especially in Group IV. In conclusion, PCV2 infection before or after vaccination with CSF vaccine affected porcine humoral and cellular immunity and led to expression suppression and unbalance of cytokines in PBLC.