Fast liquid mixing in the flow channels at millimeter size was studied by planar laser induced fluorescence (PLIF) technique in experiments and the large eddy simulation (LES) as well. A simplified mixer model was applied, where two liquid streams impinge into each other in the confined channels. A dynamic kinetic energy sub-grid scale (SGS) model of LES was employed to simulate the fast liquid mixing. The results show that LES predictions with high-density meshes have good agreement with the experimental data in terms of the instantaneous and time-averaged concentration fields, and the probability density function (PDF) of concentration fluctuations. Hence, the mixing quality characterized by the intensity of segregation (10S) and 95% mixing time can be also predicted reasonably. The LES simulations further predict the significant effect of cross-flow angle on the mixing process. The cross-flow angle of 90 degrees gives the fastest mixing of two liquids in the millimeter channels at 1-2 ms. The mixing layer between the two liquid streams cannot be observed under the geometry with larger cross-flow angles. This indicates different mixing mechanisms compared with the mixing of two liquids flowing in parallel. Vorticity in the turbulent mixing process is discussed for a better understanding on the physical factors controlling the mixing process. (c) 2007 Elsevier Ltd. All rights reserved.