Nitrogen effects on thermal donors and shallow thermal donors in nitrogen‐doped silicon have been investigated by means of the far infrared technique at low temperature (8 K). It is evident that nitrogen can suppress the formation of the thermal donor in silicon annealed at 450 °C. Nitrogen atoms are combined with oxygen atoms to produce the oxygen‐nitrogen complexes related with the shallow thermal donors. It is believed that nitrogen interacts with the oxygen atoms to form oxygen‐nitrogen complexes so as to reduce the thermal donors. ©1995 American Institute of Physics.