It is of particular interest to characterize the frequency spectrum of signal sources at submillimeter wavelengths.The heterodyne-mixing and direct-detection(Fourier transform spectrometer,FTS) methods were introduced to measure thefrequency spectrum of a submillimeter CW source(phase-locked and tunable from 460-520 GHz).A 500-GHz supercon-ductor-insulator-superconductor(SIS) tunnel junction was employed as a mixer for the former method but a direct detectorfor the latter.The measurement results are compared.