The effects of the mixing process on the power of a supersonic flow CO chemical laser are studied numerically using a leaky stream tube approach. Downstream of the nozzle exits, the flow field is divided into three stream tubes, two stream tubes for the dissociated products of CS₂ diluted in Ar and for O₂, and a third stream tube between these two, in which mixing occurs. The numerical results clearly show the importance of the mixing enhancement. As the growth of the mixing region is reduced, the laser power is lowered significantly. Therefore, the mixing process is a crucial factor in laser power estimation.