Abstract: 3-D Navier-Stocks equation with multi species and chemical reaction process was applied with CFD method on the chemical oxygen iodine laser(COIL) model using nitrogen as diluents gas to investigate the detailed flow field and the coupling between physical and chemical process, especially the appropriate mixing technique. It is found that mixing by jet in the subsonic section is inapplicable to the N2-COIL without primary buffer gas; jet in supersonic is necessary and with the appropriate flow rate ratio, the small signal gain could reach 1.5% cm-1 in the position of cavity; the efficient area of gain is widened since the thickness of boundary layer is reduced as the result of mixture weight increase.