To elicit the size-dependent carrier ability of gold nanoparticles (GNPs), synthetic peptide corresponding to foot-and-mouth disease virus (FMDV) and influenza A virus subtype (H5N1) viral proteins is conjugated to GNPs with diameter 2-nm, 5-nm, 8-nm, 12-nm, 17-nm, 37-nm, and 50-nm. Extra cysteine is added to the C-terminus of the FMDV peptide (pFMDV) and H5N1 peptide (pH5N1) to ensure maximum conjugation. Immunization of peptide-keyhole limpet hemocyanin (KLH) conjugate was performed as control. Blood are withdrawn on week 4, 6, 8, and 10. Titers against peptide and carriers are obtained. For peptide-GNP immunization, specific binding against peptide is detected in the sera of mice injected with 2-nm, 5-nm, 8-nm, 12-nm and 17-nm GNP conjugates. Maximum binding occur with GNPs of sizes between 8-nm to 17-nm. The peptide -GNPs induce 2.17 ~ 3 fold antibody responsecompared to peptide-KLH. In particular, all sera exhibited undetectable binding against GNP, while antisera of peptide-KLH present high levels of binding activity against KLH. Deposition of GNPs in mouse spleen was evaluated by ICP-MS. The immunogenicity of peptide is correlated to amounts of GNPs accumulated in spleen. In conclusion, we show the size-dependent immunogenic properties of peptide-GNP conjugates. GNPs ranging from 8-nm to 17-nm may serve as ideal carrier to elicit focused and enhanced antibody response against a synthetic peptide.