Typical microfluide devices basically operate at low Reynolds numbers. The flowage in microchannels is entirely laminar. This letter quantifies both experimentally and numerically the diffusion across the interface between two aqueous solutions in pressure-driven laminar flow in microchannels. The relationship between the flow parameters at the inlets and the interface's location and diffusion extent is verified at high Peclet number (about 800 ̃ 8000). The experimental results accord with the CFD results very well. A quantitative conclusion of the interface location and the diffusion extent between two streams near the walls, such as described in this letter, are required for the further development of the technology of microfabrication inside microchannels using fluid flow. The features of the micro structures, which are patterned with this fabrication method, are determined by interface location and the diffusion across the interface in a great degree. In the last part of this letter we give some the experiment results about the relationship of the flow parameters and the topography of the secondary patterned micro structures.