Abstract: Phase noise and spurious spectrum are the key characters in direct digital synthesizers (DDS) applied in the high stable frequency source. Actually, the poor performance of spurious spectrum from DDS has always restrained its applications. This paper introduces a new way to analyze the spurious spectrum and phase noise generated by the phase truncation. The locations and amplitudes of spurious spectrum are precisely dtermined. Phase noise generated by the size of ROM look-up table and the performance of DAC is also investigated.