Abstract: The Villermaux-Dushman fast parallel competing reaction was used to determine quantitatively the micro-mixing performance in the T-shaped micromixer with 600 μm width and 300 μm depth. The flow was almost ideal plug flow in the Reynolds number range 55—1600 and its corresponding Péclet number ranged from 7.0×10⁶ to 2.0×10⁸.The effects of inlet and outlet scale, micro-channel length, volumetric flow ratio on the micro-mixing performance were studied in detail.The micro-mixing performance was expressed with segregation index, X_s.The smaller the X_s,the better the micro-mixing.The experimental results showed that X_s increased with increasing volumetric flow ratio at the same Re.There existed a critical Re_c under the condition of different inlet and outlet geometric structures.X_s decreased with increasing Re when Re<Re_c, while X_s reached about 2.7×10^-4 and remained constant when Re>Re_c, indicating that almost perfect mixing in the T-sharped micro-mixer was achieved.