The syntheses of gold nanoclusters, namely, the 25-gold-atom nanocluster (Au25(SR)18) and 38-gold-atom nanocluster (Au38(SR)24), with SR representing the thiol ligand, were described in previously reported studies. The synthesis was via a fast reduction process using sodium borohydride. The ratio of Au:HSR:NaBH4 was 1:3:10. Herein we report that the Au25 nanocluster can also be synthesized via a slow reduction process through a dropwise addition of an aqueous solution of sodium borohydride. The ratio of Au:S:NaBH4 is also changed to 1:3:5. This method synthesized Au25 nanoclusters at a high yield (47%). Pure Au25 nanoclusters were obtained after extraction, and the product was fully characterized by UV-vis spectroscopy, thermogravimetric analysis (TGA) and Matrix-Assisted Laser Desorption/Ionization (MALDI) mass spectrometry. The possible formation mechanism is discussed in this paper. This work contributes to a better understanding of the mechanism of Au25 formation and provides a basis for further study of gold nanoclusters.