This paper presents design problems of a ramengine combustor which is supposed to operate in the flight speed range between Mach numbers 3 to 5. Two types of model combustors were tested. Main aims were to examine their flame holding ability and to measure the pollutant emission along the predicted operational conditions. It was found that both combustors, satisfying required pressure loss, could not sustain stable flame at flight Mach number 3 condition. The improvement strategy is discussed. NO_x emission was measured at temperatures ranging up to 1050K in electrically heated air. An experimentally derived correlation was used to predict NO_x emission levels for the cruise flight condition of the engine. Present results lead to the conclusion that homogeneous combustion is a prerequisite for improving the combustor performance as well as for reducing NO_x emission.