γ-PGA is a versatile multi-functional biopolymer possessing much useful biological functionality. Each biological function is associated with specific properties related to various forms of γ-PGA. However, to date, knowledge about the molecular mechanism to control specific structures of γ-PGA under a certain environmental conditions are unknown. In this study, we used single molecule force spectroscopy (SMFS) method to characterize the structure of γ-PGA in two environmental conditions. A water environment or 0.5 M MgCl2 salt solution was used to observe the effect of metal ion on γ-PGA structure at room temperature. The obtained results revealed that γ-PGA exists branched and/or cross-linked structure of γ-PGA in water or MgCl2 salt solution.