Multiplexer/demultiplexer′s impact on optical network system is studied. 10 Gbit/s optical system′s degradation caused by optical nodes is identified. Noise distribution is assumed as a Gauss one. The transmission function of optical node is considered as a band pass function. The central region of this band pass function is always not as flat as the ideal function. It causes the degradation of system performance severely. The system penalty is 2 dB～6 dB on the condition of 0.2 dB～0.5 dB variation. Also the difference of the flaw makes discrimination of penalty. The discrimination is 1 dB～5 dB considering the amplitude and rank of variation.