We demonstrate a simple and reproducible way to produce quasi-spherical Au nanoparticles (NPs) with a fairly narrow size distribution in water by rapidly adding a mixture solution HAuCl4, sodium citrate and a trace amount of silver nitrate into boiling water. The sizes of quasi-spherical Au NPs obtained increases from 12 +/- 1 nm to 18 +/- 3, 25 +/- 3, and 36 +/- 3 nm with decrease of the citrate concentration in a fairly linear way. The present protocol can efficiently minimize the effect of citrate to buffer the pH of the reaction media and thus change the type and reactive activity of auric ions and significantly speed up the nucleation and growth rate of Au NPs. The presence of Ag+ ions can not only suppress the secondary nucleation but also reshape the polycrystalline Au NPs into a quasi-spherical shape. In the case of synthesis of Au NPs of sizes ranging from 10 to 36 nm, our approach efficiently makes up the shortages of the classical Turkevich method with respect to the reproducibility and uniformity of the NP size and shape.