The purpose of the present study was to investigate fluidic behavior in a microchannel with hair-pin curves, using a three-dimensional (3-D) computational fluid dynamics simulation, and to observe the 3-D flow pattern, to validate the simulation. The microchannel used was fabricated on a PMMA plate using a flat-end mill. The channel width and depth were 210 and 205 mum, respectively, and the radius of each hair-pin curve was 500 mum. Two liquids; purified water and an aqueous solution of 50 mumol/L fluorescein, were introduced into the microchannel through different inlets and were merged, forming a side-by-side parallel flow in the straight channel. When the average velocity was 25 mm/s, the liquid was thrust outward by centrifugal force and, as a result, the vertical line that crossed the central axis was distorted after passing the first hair-pin curve. At the second hair-pin curve, the centrifugal force was exerted in the opposite direction, and the distorted line returned nearly to an initial vertical line. When the average velocity was 125 mm/s, however, the vertical line, which was distorted at the first hair-pin curve, did not recover to the initial vertical line after the second curve. The interface between the two liquids was permanently waved. The simulation was in good agreement with the experimental data. The results suggest that the diffusion rate through the interface of two liquids in microchannels with hair-pin curves can increase, compared to that in straight microchannels. (C) 2004 American Institute of Chemical Engineers.