The mechanism of film formation on the Ni anode in a well dehydrated melt of NH4F . 2HF was studied at 100 degrees C. The potentiodynamic and potentiostatic polarization behaviors of the Ni anode were investigated to elucidate the anodic processes. The oxidized layer was composed of NiF2 with a small amount of nickel oxides such as NiO and Ni2O3 or oxyfluorides having plural oxidaition states and a highly oxidized nickel fluoride formed on the Ni anode polarized at potentials higher than 4.5 V versus H-2 and it grew thicker through the repetition of the alternate formation and degradation with the time of electrolysis. The composition and the thickness of the oxidized layer may reflect on the current density on the nickel anode under polarization.