The flow field of a, supersonic flow chemical oxygen-iodine laser is simulated solving three-dimensional Navier-Stokes equations, and the dependence of the mixing/reacting zone structure and the resulting gain region on the effective velocity ratio of I₂ jet to the primary flow is studied. The I₂/He ratio and plenum pressure of the secondary flow are varied in order that the amount of iodine injected into the primary flow is kept constant in each effective velocity ratio. The present results demonstrate that a pair of contrarotating vortices generated behind the I₂ jet greatly enhances the mixing and the sumultaneous chemical reaction of I₂ and O₂(¹Δ). It is shown that the optimum condition for the secondary I₂ jet momentum exists. The I₂ jet which causes the high gain penetrates into the primary flow moderately deeply and does not collide with the counter one.