The influence of Ni ion plating 0.1 to 3.1 μm thick on the wettability of molten Al to Al₂O₃ was measured by the sessile drop method in a vacuum of 2×10⁻³ Pa at temperatures from 973 to 1173 K. The wetting angle of molten Al to Al₂O₃ decreased from 110° to 90° as the temperature rose from 973 to 1173 K. Ni coating on Al₂O₃ reduced the wetting angle to less than 10° when the thickness of coating was larger than 0.1, 0.5, and 0.8 μm at 973, 1073 and 1173 K, respectively. The thinner Ni coating could not improve the wettability, because the Ni deposits on Al₂O₃ were coagulated by surface tension and pores were generated in Ni coating. The development of pores became more remarkable as temperature rose, making the molten Al which once spread on the Ni coating move back towards the receding wetting angle to Al₂O₃. The interaction between the coated Ni and the molten Al was also metallographically investigated.