The thermal stability and formation range of amorphous Al(100-x)Ln(5)Ni(x) (x = 3, 5, 9, 13, 17, 21, 23) alloys were studied by differential scanning calorimetry and X-ray diffraction techniques. The microhardness, H-v, of these Al-RE-Ni alloys was measured. The experimental results show that toughness was adversely affected by increase in the nickel content. The atomic size factor was applied to explain the formation mechanism of aluminium-based amorphous alloys by the atomic clusters model. The minimum concentration of nickel in Al-RE-Ni alloys results from the effective atomic volume difference of the nickel and aluminium atoms.