This paper reports experimental studies of inertia fluid behaviors in contraction-expansion (C-E) array microchannels: flow separation and Dean flow. We fabricated the C-E array microchannels with the shape of rectangular and semicircular structure by using poly(dimethylsiloxane) (PDMS) molding technique. In order to observe the flow separation, we introduced 1 μm fluorescent polystyrene beads into the C-E array microchannels and captured fluorescent images of the bead behaviors. The oncoming flow separates at the exit corner of the contraction channel region and reattaches to the wall downstream generating re-circulation flows adjacent to the wall. Vortices are formed by the re-circulation flows in the C-E array microchannels when the Reynolds number (Re) of a flow exceeds a certain critical value, and a size of the vortices is expanded as the increase of Re. We characterized the inertia fluid behaviors by flow separation in the C-E array microchannels with different expansion channel lengths, contraction channel widths and channel structures. For investigating the Dean flow, two fluids of different types (FITC solution and deionized water) were injected into the C-E array microchannels. When the inertia fluid flows through the contraction regions in the C-E array microchannels, it is influenced by a centrifugal force as if the fluid travels through curvilinear channels. The centrifugal force accelerates the fluid outwards from a wall and finally causes the Dean flows. Vortices by the Dean flows were observed in the contraction regions of the C-E array channels and the vortices were influenced by change of Re and channel structures. Additionally, numerical simulations are performed to predict the inertia fluid behaviors in the C-E array microchannels and to estimate flow characteristics such as axial, transverse velocities, and pressure distributions.