We propose a grooved micromixer incorporating an overlapping crisscross inlet port that is located at the intersection of two patterned channels crossing one above another. Both numerical analysis and experimental verification of the flow structure of this design have substantiated the superior mixing features over the existing herringbone mixer. Because of the symmetric feature of this microstructure, fabrication becomes simplified through assembling two identical PDMS-based slabs oppositely. Both experimental results for flow visualization and numerical simulation reveal significant cross flow at the intersection of the two channels and vertical tumbling of the flow. This activated flow feature supplies downstream fluids with vertical momentum to enhance the chaotic advection and enlarges the interfacial area between the two mixing fluids. All these features improve the mixing performance of this novel overlapping crisscross micromixer by 46% compared with the mixing indices of the staggered herringbone mixer for the longitudinal distance L = 2047 Vm. Furthermore, on modulating the ratio of volumetric flow rates between the two inlet streams, an excellent mixing function with a specific prearranged concentration of a mixture and a decreased pressure loss are achieved. The divided ratios Q(t)/Q(t), defined as diverted flow rate over forward flow rate, are between 1.86 and 2.88 for fluid A and between 0.52 and 0.67 for fluid B with a variable initial flow rate ratio. (c) 2006 Elsevier Ltd. All rights reserved.