This paper reports a facile synthesis method for shape control of gold nanoparticles by treating an aqueous solution of chloroauric acid with sodium citrate and poly(vinyl pyrrolidone) (PVP). By such an environmentally friendly method, gold nanoparticles were prepared with different morphologies including spheres, rods, and worm-like nanoparticles. Two possible growth mechanisms were proposed in the formation of wormlike particles: from spherical colloids to nanorods, and from nanorods to worm-like particles by stacking faults or by rods conjunction. It was found that the PVP plays a key role in the formation of nanostructures by acting as reducing agent, stabilizer, and surface capping agent. Density function theory (DFT) is applied to quantify the binding energies of PVP with different gold crystal surfaces (i.e., 100, 110, and 111). Such a DFT approach, facilitated by transmission electron microscopy and ultraviolet-visible spectroscopy characterizations, will be useful for fundamental understanding of the growth mechanism of gold nanorods.