The invention relates to a method of operating a gas turbine assembly, which comprises a compressor, a combustor and a gas turbine, the method comprising steps of: operating the gas turbine assembly at a partial load with respect to a base load of the gas turbine assembly; bypassing a portion of blow-off air from the compressor; and introducing the portion of blow-off air into an exhaust gas duct after the gas turbine. When the gas turbine assembly is operating at partial load, can maintain the air-to-fuel ratio approximately constant and hence to control the combustion flame temperature at a level where emissions are kept below the permission limit.