The characteristics of mixing between two flows of oxygen and iodine, having different Mach numbers, are numerically examined with the intention of improving the pressure recovery of the exhaust gas of supersonic-flow COIL. Supersonic, parallel mixing system with ramp nozzle array is adopted. The nozzle array has unevenly-piled shape, which is expected to generate vortices and enhance mixing. Three-dimensional, compressible Navier-Stokes equations are solved by means of full-implicit finite difference method. The flow fields are calculated for three types of nozzles, namely, ramp nozzle, symmetric ramp nozzle, and symmetric swept-ramp nozzle. The results indicate that the symmetric swept-ramp nozzle has the best performance.