A one-step, size control synthesis of gold nanoparticles under mild conditions by reducing HAuCl 4 with 3-thiopheneacetic acid (TA) was introduced and investigated as a function of TA concentration and molar ratio of TA to gold. Here, TA also acted as a protecting agent by loosely absorbing on the surface of produced Au 0 . With a relative higher concentration of TA or under a lower temperature, only spherical gold nanoparticles were formed. While lowering the concentration of TA, increasing the molar ratio of TA to gold, or increasing the reaction temperature, larger gold particles both in spherical and polygonal could be obtained. At the same time, the formation of gold nanoparticles with TA was also investigated by using in situ UV–vis absorption spectroscopy and transmission electron microscopy (TEM).