The N-doped and N-undoped single crystal silicon under different conditions was investigated by means of Fourier Transfosm Infrared Spectroscope (FTIR) at room temperature (300K) and low temperature (SK). The experiments pointed out that the 1030cm-1, 1000cm-1 and 806cm-1 optical absorption lines are related to the nitrogenoxygen complex in nitrogen-doped CZ silicon.