This paper presents a numerical study of the structure flow and heal transfer in the discharge tube of a high power CO2 laser. A compressible turbulent model was developed in connection with simplifying assumptions. The resulting set of partial differential equations describing the flow was solved by means of the PHOENICS code. Several types of thermal boundary conditions were tested and the numerical results were found in a quite good agreement with the experimental data obtained in a previous study. The location of the reattachment point was found highly correlated with the values of the turbulent energy and the dissipation rate of turbulent energy in the entrance section. The size of the recirculation zone and the shock waves created near the exit section of the nozzle have no significant effect upon the structure flow downstream of the reattachment point.