Miniaturized devices referred to as microchannels produce special and outstanding chemical reaction behavior; however their mechanism and their fluidic behavior, which is indispensable to analysis of the mechanism, have not been elucidated. This work performs three-dimensional observation of fluid using confocal fluorescence microscopy and computational fluid dynamics simulation to assess the fluidic behavior in curved microchannels. Results showed that a complicated differential medium interface between aqueous liquid/liquid is produced by the laminar secondary flow at the curve of the microchannel. The interface area increased more than three times over that of the flat simple interface. Density and viscosity of the fluid also affect interface configuration, and the results suggest that only the Reynolds number is insufficient to account for the fluid behavior in curved microchannels. Drastic variation in the interface area increases diffusive mixing: it also shows that the understanding of fluid motion and mixing behavior is necessary to design and manufacture microreactors.