To better understand the mass transfer and provide possible performance improvement methods for the microfluidic devices relying on the two-layer laminar flow, a computational fluid dynamics simulation is performed to investigate the phenomenon of a two-layer laminar stream passing through a curved microchannel. The results show that due to the density difference between the two fluids, asymmetric pair of Dean vortices are formed as the fluids pass through the curve. Due to the inter-stream convection, the denser fluid intrudes into the less-dense one resulting in a Gamma sharp layer-interface. Furthermore, the parametric analysis helps identify ways to minimize the inter-stream mass transfer, such as increasing the density ratio, as well as reducing the Re number and aspect ratio. The study concludes that the microchannel curve can be properly designed to obtain a clear interface between two streams to fulfill the basic requirement of microfluidic devices. (C) 2011 Elsevier B.V. All rights reserved.