Abstract: Numerical simulation of chemical oxygen-iodine laser(COIL) flows with nitrogen diluent based on RADICL setup was carried out. A finite-volume code was used to solve 3D compressible N-S equations with 10 components, 13 non-equilibrium chemical reactions. The condition for the secondary flow to match the primary flow was obtained, and the proper penetration-depth of the secondary flow was found. As a result, the rate between the secondary and the primary dynamic pressures was about 13.0. Simulation showed that gases flow slower and stay longer in the region of the nozzle and the iodine molecules dissociate faster with nitrogen diluent than that with helium diluent. It was found that the gain has a big peak near the throat of the nozzle, and decreases rapidly downstream. It showed that moving