Gold nanoparticles are prepared in O/W microemulsion of SDS/Vc/H2O (0.001 mol?L-1 HAuCl4) system by using Vc as the reducing agent. The size of obtained polygonal gold nanoparticles is from 5 nm to 20 nm,and increases with the content increase of HAuCl4 in O/W microemulsion in the condition of SDS/Vc weight ratio of 50/50. The formation mechanism of gold nanoparticles is discussed in the present paper.