Two-dimensional viscous laminar incompressible flow has been studied for 90 degree curved channels. The channel of constant width is suddenly expanded, either inward or outward, at the middle of the 90 degree turn. The separated flow is solved numerically using a finite difference method. Velocity profiles, streamlines and vorticity distibutions are obtained for Reynolds numbers 100,300 and 500. In addition, flow in a 90 degree turn channel without expansion is calculated to provide a basic knowledge on the effect of the sudden expansion. Flow visualization experiment has also been conducted for the same geometry to confirm the computational results.