Active catalysts for the direct decomposition of nitrogen monoxide were prepared from amorphous Ni-40Ta-Pd alloys by HF-treatment and subsequent pre-oxidation. The pre-oxidized catalyst consists of NiO, Ta2O5 and PdO, and PdO is decomposed to fee Pd by heating at temperatures higher than 600 degrees C. At further higher temperatures NiO and Ta2O5 are transformed to a very fine gained double oxide NiTa2O6. The catalysts thus formed consist of three layers and show high catalytic activity for the decomposition of nitrogen monoxide in a wide temperature range from 550 to 850 degrees C. The catalytic behavior is affected by the structural change in the catalyst. With the transformation from NiO and Ta2O5 to NiTa2O6, the catalytic activity and the nitrogen formation selectivity increase significantly. TEM observation of ultramicrotomed cross-sections revealed that finely dispersed palladium supported on very fine-grained NiTa2O6 is formed in the interface between the outer and intermediate layers. High catalytic activities and high nitrogen formation selectivities of the Ni-40Ta-Pd alloys are attributable to the formation of the NiTa2O6-supported palladium catalyst.