The mixing is difficult but fundamentally important in microfluidic systems. Three simple yet novel passive mixers were designed and fabricated with cylindrical grooves adjoining to the main straight channel. A serial of systematic simulations and experiments have been done with the increase of the groove-depth, ranging from 0 to 3/4-cylindrical groove. All results confirmed the improvement of mixing in these mixers in a short distance (<20 mm). Over a wide range of Reynolds number (from 0.1 to 100), the ratios of 'the percentage of mixing (phi)' of the mixer with 1/4-cylindrical grooves to that with no grooves increase from 1.23 to 5.11. All designs are very easy to fabricate and have great potential to be integrated into a total microfluidic system. (c) 2012 Elsevier Ltd. All rights reserved.