The flow field of a nozzle starting process with thermal and chemical nonequillbrlium has been simulated. This flow is produced in high enthalpy impulse facilities such as the free piston shock tunnel. The governing equations are the axisymmetric, compressible Navier-Stokes equations. In this study Par's two-temperature model.. where air consists of five species, is used for defining the thermodynamic properties of air as a driven gas. The numerical scheme employed here is the hybrid scheme of the explicit and implicit methods. which was developed in our laboratory. along with AUSM(divided by) to evaluate inviscid fluxes. In the present simulation, the Mach number of an incident shock wave is set at M-s = 10.0. It corresponds to a specific enthalpy, h(0), of 12 MJ/kg. The results clearly show the complicated thermal and chemical nonequilibrium flow field around the end of the shock tube section and at the nozzle inlet during the initial stage of the nozzle starting process. They also suggest that the phenomenon of nozzle melting might be associated with a flow separation at the nozzle inlet. (C) 2004 Elsevier Ltd. All rights reserved.