Fluidic systems usually contain multiple channels connected together, and the blood system or the lungs are typical examples. Microdevices consisting of a pair of microchannels in series, integrated with pressure sensors, were fabricated to study fluid flow in such complex microsystems. The dimensions of the channels are about 40μm×1.2μm×2000μm for the wide channel and about 20μm×1.2μm×2000μm for the narrow one. The channels are connected via an expansion/contraction section with an included angle of either 15° or 90°. Nitrogen was passed through the micro devices under inlet pressures up to 50psi. Each device was tested in the expansion, flow from narrow to wide channel, and contraction mode, flow from wide to narrow channel. Mass flow rate was first measured as a function of the overall pressure drop. Then the detailed pressure distribution along the channels was measured to understand the flow pattern around the expansion/contraction section. The reduced Reynolds number for such flows is about 0.001, suggesting that the flow is of the Hele-Shaw type with no separation. However, our results show that the flow resistance is higher than the friction losses, indicating that the flow may separate at the transition between the channels, and the added resistance due to the separated flow is not negligible although the channels are very long.