Anti-idiotype (Id) antibodies can be used to induce specific cellular immune responses against tumor antigens, but the mechanism of antigenicity is not always clear. We previously reported an anti-Id antibody, 6B11, which mimics human ovarian cancer associated antigen OC166-9. To explore the molecular basis of cellular immune response induced by 6B11, a panel of peptides derived from complementarity determining region (CDR) of 6B11 were synthesized. After a series of immunologic experiments, we found that the light chain CDR3 peptide and heavy chain CDR3 peptide were the MHC class I and class II epitopes of 6B11, respectively. The combination of MHC class I and class II epitopes is more effective than 6B11 in inducing specific cellular immune response against ovarian cancer. Our study provided the structural basis of antigenicity of 6B11. The identification of antigen-specific T cell eptitopes in 6B11 should facilitate the design of epitope-based vaccine against human ovarian cancer.