Rod-shaped gold nanoparticles were prepared by a seed-mediated growth method with Ag+ as assistant ion, CTAB as surfactant and ascorbic acid as weak reductant. The effect of amount of seeds on the rod aspect ratio and the yield was studied. The morphology and crystal structure of rod-shaped gold nanoparticles were characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The optical properties were tested by UV-Vis spectroscopy. The results indicate that the rod-shaped Au particles were fcc structure. UV-Vis absorption spectra show a transverse plasmon absorption band at 530 nm and a longitude plasmon absorption band at 970-980 nm. Aspect ratios of the rod-shaped nanoparticles increased first, and then decreased with the amount of Au seeds increasing. Meanwhile, an increase in the proportion of spherical gold nanoparticles was observed when the Au seeds were added excessively. Finally, the growth mechanism of the products was discussed.