In the present work, both the strength relationship between the absorption lines concerning nitrogen and the variations of line strength with heat-treatment conditions were studied by means of Fourier transform infrared spectroscopic techniques (FTIR). A structural mode for carbon-oxygen complexes and nitrogen-oxygen complexes in silicon is put forward for the analyses of the experimental facts combined with the previous work of other investigators. It offers a satisfactory explanation to all the experimental facts concerning carbon-oxygen complexes and nitrogen-oxygen complexes.