A microfluidic system offers superior controllability of fluids. This study developed a novel separation method for complexes formed in a microchannel. It presents an application for DNA sensor. This method is based on the molecular sieving effect, which is achieved by superior controllability of fluidics. Use of confocal fluorescence microscopy observation and computer simulation confirmed that the laminar secondary flow at the curing part of a microchannel enables this microfluidic separation. Also in this study, we applied this molecular sieving effect for sequence-selective DNA sensor. This measurement showed that the response of a complementary sequence target DNA was higher than that of a non-complementary sequence. Moreover, the response of a longer target was higher than a different-length complementary sequence target. This method does not require immobilization of probe or target DNA: solutions are merely injected into the microchannel and all reactions occur in the liquid phase. Such features might lower the experimental error and difference in data by operators. (C) 2003 Elsevier B.V. All rights reserved.