Influence of perturbation frequency on mixing flow velocity and the varying flow structure in T-shape microchannels are investigated using laser induced fluorescence. The external disturbances of flow are created excitation system based on the piezoelectric actuator. Several increases in mixing efficiency were common to all regimes, but also some different features, were observed. For the stationary vortex flow regime, we obtained a decrease in mixing efficiency of 10% at a frequency of 650Hz. For the stationary asymmetric vortex flow regime, the mixing efficiency increases by 33% and 23% for the frequencies 500Hz and 800 Hz, respectively. For Re=400 and Re=300, the flow structures without disturbance are virtually identical; however, in the case of external influence for the quasiperiodic unsteady flow regime, the mixing efficiency decreases by 22% and 29% at distances of 1 and 5 calibres, respectively, for frequency of 1000 Hz.