Nanoparticles conjugated with DNA ligands possess 1) assembly behavior that may be controlled by DNA length and sequence, as well as the presence of other ligands, and 2) material properties determined by the nanoparticle size, shape, and chemical identity. The combination of these features could allow for the production of novel materials with unique or useful properties. Previous work has demonstrated the application of DNA-capped gold nanoparticles for colorimetric DNA detection, and ordered assemblies of DNA-capped gold nanoparticles have also been reported. Here, small-angle x-ray scattering (SAXS) from a synchrotron source was used to characterize ordered assemblies of DNA-capped gold nanoparticles. The extracted structure factor obtained from these diffraction experiments confirmed the assembly of gold nanoparticles into crystallites with well-defined internal order, demonstrating the formation of a face-centered cubic (FCC) crystal lattice structure. Applying basic principles of x-ray diffraction such as the Scherrer Formula, the interparticle spacing and average size of crystallite aggregates were determined. The experimental approach outlined in this work may lead to the synthesis of programmable materials with unusual optical or electrical properties or controllable self-assembled topology. The ultimate goal of research in these types of DNA-capped colloidal systems is the development of a modular toolkit approach for nanoparticle self assembly.