Abstract: Aluminum oxide with high resistivity and tritium permeation resistance is considered as a promising candidate functional material for ITER. In this study, aluminum oxide coatings were prepared onto the China Low Activation Martensitic (CLAM) steel substrates via RF magnetron sputtering. The structure and the surface morphologies of the coatings prepared at various O₂/Ar ratios were characterized via grazing incidence X-ray diffraction, Raman spectroscopy and atomic force microscopy. The volume resistivities of the coatings were measured. The hydrogen absorption of the coating samples was also studied. The results show that the aluminum oxide coatings prepared at O₂/Ar ratios of 0.1 and 0.5 are amorphous, while the aluminum oxide coating prepared at an O₂/Ar ratio of 0.4 has a poor δ phase crystalline structure. The roughness average and the particle size of the coatings prepared at O₂/Ar ratios of 0.1 and 0.4 are smaller than those prepared at an O₂/Ar ratio of 0.5. The volume resistivities of the coatings prepared at various O₂/Ar ratios all exceed 2.7×10¹⁴ Ω•cm, and the coating prepared at an O₂/Ar ratio of 0.4 has the largest resistivity, reaching 2.1×10¹⁵ Ω•cm. The hydrogen absorption of the coating prepared at an O₂/Ar ratio of 0.5 is the smallest. The electrical insulation and hydrogen absorption properties of the aluminum oxide coatings are affected significantly by O₂/Ar ratios.