Powder X-ray diffraction (XRD) is employed to identify the phase structure of Al-substituted nickel hydroxide, and its structural chan-e after ageing treatment or charge/discharge cycles. The cell constants and the crystalline size of nickel hydroxides have been obtained by analysing the XRD pattern with Si as an internal standard. Results show that Al-substituted alpha-phase nickel hydroxide is very stable in strong alkaline medium even after 100 charge/discharge cycles.