An assembly of gold nanoparticle through the recognition of unmodified antibody was developed. The use of peptide (Cys-Ala-Leu-Asn-Asn) as ligands to stabilize and functionalize gold nanoparticles provides technical and operational convenience. These peptide-capped particles in different sizes are recognized by antibody and assembly to form dimers and expanded hybrid material by controlling the conditions. The interparticle spacing of these assemblies was well studied with small-angle X-ray scattering measurements, and it was found that the interparticle spacing is inversely dependent on the particle size. This relationship of interparticle spacing and particle size is closely related to the structure of antibody linker. Therefore, analyzing the interparticle spacing of assemblies can reveal the equilibrium configuration of IgG. Based on the investigation, the Fab-Fab angle of IgG is obtained to be ≈102° and the Fab arms are ≈7.8 nm. These results provide new experimental data on the structure of flexible IgG.