Abstract: Numerical simulations are presented for flows about a circular cylinder which is spanwise placed at the center of plane mixing layers. A temporally and spatially third-order finite differencing scheme is used to solve the 2-D incompressible Navier-Stokes and continuity equations. The Reynolds number for computation is fixed at Re=1000. The effects of the velocity ratio Ra and the momentum thickness θ of the mixing layer on flow structures are studied by changing Ra from 0.0 to 1.0 and θ from 0.2 to 2.0.