A seed-mediated method was employed here for CTAB-assisted gold nanoparticle growth. 3–4 nm silver aqueous colloid was stabilized by sodium citrate and used as seed solution to initial gold particle growth. The concentration of seed solution was calculated based on its relationship with silver atom concentration and seed particle statistical mean volume. It was found that there is a maximum seed concentration of 8.57 × 10−12 M (~25 µl 0.343 × 10−8 M seed solution added) in 10 ml 2.5 × 10−4 M HAuCl4 growth solution for growth of rodlike particles. Below this seed amount, the aspect ratio of nanorods could be controlled by varying the silver seed amount, i.e. nanorods with aspect ratio ~18.9 were obtained when the seed concentration in the growth solution was 0.343 × 10−12 M by adding 1 µl 0.343 × 10−8 M silver seed solution and nanorods with aspect ratio ~9.69 were obtained when the seed concentration in the growth solution was 1.715 × 10−12 M by adding 5 µl 0.343 × 10−8 M silver seed solution. As the seed concentration in the growth solution was more than 8.58 × 10−12 M (25 µl 0.343 × 10−8 M silver seed solution was added), there were no rodlike particles formed but spherical ones instead. These spheres were further studied by TEM and found to all be hollow structures. It was suggested that there were probably two different nucleation processes for growth of nanorods and spheres. For hollow spheres, the reaction between Ag seeds and Au ions formed hollow structures based on the Ag particle template effect. Then further growth of Au on these hollow structures produced hollow gold nanospheres. For nanorods, due to the very low concentration of silver seed (molar ratio of Ag seed: Au = 3.426 × 10−8), the growth process here probably was started by silver-induced Au nucleation, in which reduction of gold ions by silver resulted in small gold clusters. These gold clusters further grew up into nanoparticles and nanorods in the presence of CTAB.