A novel strategy of dual steric hindrance, which was obtained by Janus modification of gold nanoparticles (Au NPs) and volume exclusion of DNA, was adopted to prepare mono-DNA-modified Au NPs. The yield of mono-DNA-functionalized Au NPs significantly improved from 44 to 70% in the reaction between Au NPs and thiolated DNA. Furthermore, the specificity of mono-DNA-functionalized Au NPs was enhanced from 57 to 95%. The as-prepared Au NPs without postsynthetic treatment showed good controllability in self-assembly fabrication of complex nanostructures.