The thermal stability of Co/C soft x-ray multilayers is improved by 100-200 centigrade degrees through doping with N. The low-angle x-ray diffraction of CoN/CN soft x-ray multilayers indicates that their period expansion is only 4% at 400℃, and the interface pattern of the multilayers still exists even if they are annealed at 700℃. High-angle x-ray diffraction and transmission electron microscopy analyses reveal that this crystalline process is significantly retarded by doping with N atoms, leading to a smaller grain size at higher annealing temperatures. Raman spectroscopy analyses of the multilayers give evidence that the formation of the sp3 bonding is suppressed effectively by doping with N atoms, and thus the period expansion is decreased considerably. The strong covalent bonding between N atoms and the ionic bonding between Co and N atoms can slow down the structural relaxation. The significant suppression of grain growth is believed to be attributable to the coexistence of hcp and fee Co structures at annealing temperatures higher than 500℃.