Nickel hydroxide samples were prepared by using sodium hydroxide and ammonium hydroxide as precipitating agents. The powder X-ray diffraction pattern shows that the degrees of crystallinity in these samples are quite different, The thermal decomposition mechanism of these two nickel hydroxide samples has been determined using powder X-ray diffraction and thermogravimetric analysis. We observe that the transformation of nickel hydroxide to nickel oxide in the crystalline sample is via a two-phase mixture, whereas in a poorly ordered sample, it is through a single phase. This indicates that the decomposition mechanism mainly depends on the preparative conditions and the nature of the sample.