Preconcentration microfluidic devices are fabricated incorporating straight or convergent-divergent microchannels and hydrogel or Nafion membranes. Sample preconcentration is achieved utilizing concentration-polarization effects. The effects of the microchannel geometry on the preconcentration intensity are systematically examined. It is shown that for the preconcentrator with the straight microchannel, the time required to achieve a satisfactory preconcentration intensity increases with an increasing channel depth. For the convergent-divergent microchannel, the preconcentration intensity increases with a reducing convergent channel width. Comparing the preconcentration performance of the two different microchannel configurations, it is found that for an equivalent width of the main microchannel, the concentration effect in the convergent-divergent microchannel is faster than that in the straight microchannel.