Strain localization occurs under different geological scales and in both the brittle and viscous regimes, as the one of most significant processes of solid earth. The earliest stages of strain concentration essential to determining the cause(s) of localization. Bridge zone, or "interconnection of weak phases", occurs in less deformed, which reveals initialization of localization. These “bridge zones” comprise reduced grain sizes and an aggregation of relatively fine grains in a narrow band. Combined optical, electron beam petrography with cathodoluminescence (CL) and electron backscatter diffraction (EBSD) are used to characterize these “bridge zones”: 1. Bridge zones result from not only in-situ grain size reduction (such as recrystallization or cataclasis), but some chemical processes (not major elements) for phase mixing or element discharging on a short spatial scale; 2. The Bridge zone can be observed in wide range of P-T conditions; 3. Bridge zone is an aggregation of relatively fine grains in a narrow band and mechanically links rheologically weak phases or domains, which is weaker than the domains it replaced, and efficiently reduces the bulk strength of rock.