Regulation and design of material structure is very attractive and important for achieving special properties. Oxidative etching can be used to design nanomaterials with a variety of structures. It is crucial to reveal the structural evolution and etching mechanism. Here, the etching processes of gold nanobipyramids (Au NBPs) are observed in the presence of O2 and Cl– ions serving as oxidative etchants by in situ liquid cell transmission electron microscopy (TEM). By controlling [AuCl4]− ions, two types of etching pathways are characterized: (1) a fast nonequilibrium oxidative etching process during which Cl– ions are uniformly adsorbed on the surface of Au NBPs and a rod intermediate is formed that eventually dissolves in aqueous solutions with only HCl and (2) a slow equilibrium oxidative etching process during which atomic etching takes place at tips or edges and a small amount of Au NBPs is dissolved to reach a dynamic equilibrium with the addition of HAuCl4. The addition of ions to adjust the material structure can provide guidance for material design and application.