The pyre lytic behaviour of Nicalon under a N-2 atmosphere was investigated at temperatures from 1673 to 1973 K, and was compared with that under an Ar atmosphere. The pyrolytic rate was measured by thermogravimetry, and heat-treated Nicalon was examined by X-ray diffraction, scanning electron microscopy, Auger electron spectroscopy and tensile testing. The pyrolytic rate was smaller in N-2 than in Ar. The nitrided case retarded the crystallization into beta-SiC and retained its high strength. The effectiveness of the nitrided case disappeared on heating in Ar. The strength was related to the size of the beta-SiC crystal in Nicalon.