The size-selective microfluidic separation of glass beads in a curved rectangular microchannel was fabricated in our previous work. In this study, we improve its separation performance and attempt an experimental visualization to examine the separation resolution. In the previous work, we found by visualization that the trajectory of 20 µm glass beads was influenced by their travelling path along a straight inlet channel. Using a forced sheath flow, a consistent bead trajectory along the middle of the straight inlet channel was obtained, and the sheath angle to minimize the focusing width of the flowing distributed beads was determined to be 45°. The physical explanation for the dynamic behavior of microbeads was elaborated. When the ratio of Stokes force to centrifugal force mainly acting on a glass bead fell under unity, the glass bead moved out to the wall in spite of the fact that its size was less than the height of the zero velocity position. To examine the separation resolution, the newly designed size-selective separation microchannel with the sheath was fabricated and its separation performance was visualized. The movement of the glass beads showed a good agreement with the separation mechanism explained by the force ratio. The resolution of the separation was visualized to be 10 µm for the size of glass beads used in the experiment. The size-selective separation performance was explained in terms of physical forces and was improved by solving the previous problems. A cascade device for the continuous separation of microbeads of various sizes can improve the separation resolution.