In this paper, a coral-shaped three-dimensional (3D) assembly of gold nuclei was formed by UV irradiation of HAuCl4 solution in the presence ofcitric acid. Such coral-shaped nanostructure was built up of numerous nuclei less than 5 nm in diameter. The results showed that the initially formednuclei spontaneously experienced a self-assembly process to form this 3D nanostructure. Transmission electron microscopy (TEM) images indicatedthat this coral-shaped 3D nanostructure was disintegrated during the ripening process. The UV irradiation-assisted ripening process induced theformation of single-crystalline nanonetworks. However, when the ripening process was performed without UV irradiation, the nanoparticles withvarious shapes were observed. UV-vis spectra indicated that the unreduced AuCl4− was responsible for the formation of the coral-shaped 3Dnanostructure.