Abstract A modelling study is performed to investigate the plasma flow through the constrictor of medium-power arc-heated thrusters. The Roe scheme is employed to solve the governing equations, which take into account the effects of compressibility, Joule heating and the Lorentz force, as well as the temperature and pressure dependence of the gas properties. The modelling results show that the gas flow within the constrictor is choked by the combined effects of friction and arc heating. The computed total pressure drop within the constrictor due to friction and arc heating is about twice the dynamic head at the constrictor outlet. The effects of the arc current, inlet pressure and mass flow rate, and the constrictor length and diameter on the gas flow characteristics within the constrictor are presented and discussed. Further, it is shown that the choking effects caused by arc heating reduce the flow rate more strongly for argon and nitrogen working gases than for hydrogen working gas.