We theoretically and experimentally demonstrate the generation of flat millimeter-wave noises by optically mixing multiple Gaussian wavelength-sliced ASE lights. The proof-of-principle experiment has shown the effectiveness of our proposed method with a satisfactory relative flatness as low as 0.46 [@(35±15)GHz]. Considering the non-dependency of fibers loss on frequency and the use of a photodetector with faster response such as uni-traveling-carrier photodiode (UTC-PD), the experimental implementation of our method in a higher frequency band can be expected. Our proposed method exhibits potential features for the noise parameter measurement of millimeter-wave devices.