A Mach 2 hydrogen - air supersonic combustor model with no diffuser was designed and operated in atmospheric inflow condition. A new supersonic mixer, which was vent slot mixer (VSM), was developed and fabricated to use as the device of the fuel-air mixing. At low enthalpy inflow condition, a plasma jet torch was used as the igniter and the flame-holder. An isolator was located between the nozzle and the combustor. In supersonic combustion, the VSM had an effect to increase the mixing efficiency compared with the step mixer. While the combustion pressure was increased, the combustion mode was changed from unstable supersonic combustion to dual-mode transition. When the unstable flow of shock train affected the combustor inlet, the VSM was showed that the combustion stability was slightly sustained when compared with the case ofthe step mixer.