A synthesis procedure of gold-silica core-shell nanoparticles has been developed in this work. We have investigated the effects of the amounts of poly(vinylpyrrolidone) (PVP), water, and tetraethoxysilane (TEOS) in the starting materials on the shell thickness. The results reveal that the modification of the surface of gold nanoparticles with PVP plays a critical role in the formation of smooth coating with silica. The shell thickness can be well controlled by changing the amounts of water and TEOS, typically ranging from 2 to 25 nm. We have examined optical extinction properties experimentally and numerically; our experimental results are consistent with the calculations.