Comparison was made for the citrate stabilized gold colloids prepared sonochemically and thermally. In the sonochemical reduction system, purge gas had large influence on the formation of gold nanoparticles, while little effect was observed for the thermal reduction system. The average diameter of sonochemically prepared gold nanoparticles was smaller than that of thermally prepared gold nanoparticles when prepared at the same concentrations of HAuCl4 and Na-3 citrate. The average diameter of the sonochemically prepared gold nanoparticles was increased with an increase in the HAuCl4 concentration ([Na-3 citrate]/[HAuCl4 = 7), while the average diameter of the thermally prepared gold colloid was independent of the HAuCl4 concentration. These results were explained by the difference in the reduction rate of gold ions.