Abstract: The surface morphology,crystallite size distribution, and the surface and depth analyses for the coneentratons of impurity elements and cobalt oxide ions of several samples of Co_3O_4 catalyst for ammonia oxidation, which were obtained under various conditions for preparation, calination,usage,and regeneration, were observed by means of a transmission electron microscopy(TEM), a scanning electron microscopy(SEM),and an ion microprobe mass analyzer (IMMA). The observed results are discussed in relating with the experimental activity of the catalyt. It is concluded that the possible causes of the decline in the activity of the catalyst are that the impurity elements transport from the bulk phase and enrich onto the surface and the cobalt oxide ions on the surface decrease when the catalyst has been used at the high temperature for a long time.