The change in composition of the surface layer of beta-Si3N4 whiskers was examined after heat treatment in atmosphere. At 873 K, the beta-Si3N4 whisker was barely oxidized. At 1273 K, the oxidation of the surface layers of the whisker occurred easily. With the beta-Si3N4 oxidation, the Si-N bond gradually changed into the Si-N-O bond, and finally became the oxidized layer (amorphous layer) of the whisker surface. It was assumed that the whisker surface has a gradient interface structure which gradually changes from the oxide layer of the whisker's outer surface to the nitride crystal of the inside layer. It was confirmed that impurity elements such as Y and Ca existed mainly in the amorphous region near the interface between the amorphous layer and the crystal layer.