A micromixing scale is proposed to characterize the micromixing performance of microdevices. It is defined to be the smallest scale within which homogenization is realized only by molecular diffusion, indicating the homogeneous mixing ability of a certain microdevice under specific operating conditions. An approach that integrates numerical simulation with experimental measurement was developed to establish the correlation between micromixing scale and segregation index. The Villermaux-Dushman reaction was conducted in a T-junction micromixer to determine its micromixing scales. The validity and reliability of this new approach is demonstrated by employing another parallel competing reaction based on both the simulation results and the experimental data.