The entire flowfield in a segmented-constrictor type arc-heated wind tunnel is simulated to numerically characterize the freestream properties produced by the wind tunnel. The flow from the upstream electrode chamber to the nozzle throat is solved using a newly developed Navier-Stokes code named ARCFLO3 which accounts for Joule heating and radiative transfer phenomena in the arc heater, and the downstream region of the nozzle beyond the throat is described by thermochemical nonequilibrium Navier-Stokes equations using the flow properties at the nozzle throat obtained by the ARCFLO3 calculation as an inflow condition. The method is applied to calculate one operating condition in the 1MW arc-heated wind tunnel facility at ISAS. The calculated results are compared with the experimental data to show the validity of the present method.