Rod-like gold nanoparticles can be formed from HAuCl4 aqueous solution with ultraviolet (UV) irradiation in the absence of templates, avoiding possible poisonous surfactant embedded into the surface of functional gold nanoparticles applied in biomedicine. The aspect ratio of nanorods ranges from 1.1 to 4.4 with the concentration of HAuCl4 0.2 mM, which can be verified by HRTEM images and the absorption spectrum. It is found that the nanocluster with several atoms occurs prior to the formation of nanorods, highly corresponding to its typical absorption spectrum. Tracing the evolution in absorption spectra of HAuCl4 aqueous solution, it can be concluded that the formation mechanism of gold nanoparticles in this process is different from that in the conventional inductive growth model, possibly in a growth pattern with imperfect oriented attachment (IOA) relevant to the colliding-and-growing. Rod-like gold nanoparticles can be formed from HAuCl4 aqueous solution with ultraviolet (UV) irradiation in the absence of templates, avoiding possible poisonous surfactant embedded into the surface of functional gold nanoparticles applied in biomedicine. The aspect ratio of nanorods ranges from 1.1 to 4.4 with the concentration of HAuCl4 0.2 mM, which can be verified by HRTEM images and the absorption spectrum. It is found that the nanocluster with several atoms occurs prior to the formation of nanorods, highly corresponding to its typical absorption spectrum. Tracing the evolution in absorption spectra of HAuCl4 aqueous solution, it can be concluded that the formation mechanism of gold nanoparticles in this process is different from that in the conventional inductive growth model, possibly in a growth pattern with imperfect oriented attachment (IOA) relevant to the colliding-and- growing.