As a classic analytical technique,X-ray diffraction （XRD） method is applied in a board range of fields,such as phase analysis,grain size calculation,structure analysis,etc.The grain size is important to characterize the physical and mechanical properties of alumina materials.Here,the grain size of nano-materials based on the XRD characteristic technique.Firstly,the calcined alumina sample was qualitatively analyzed by XRD,and the α crystal structure of the sample was confirmed by matching with the standard pattern.Gaussian distribution method and Hall method were used to fit the broadening values caused by sample factors（grain refinement and micro strain）.The instrument width value at any angle was calculated according to the diffraction fitting results of the NIST LaB₆ powder and the broadening influence was eliminated.Compared with the grain size values of each crystal plane obtained by the Scherrer formula,the calculated results indicated that the Hall method value was more accurate and the broadening function caused by grain refinement and micro-strain follows the Cauchy function.It was confirmed that the average grain size of the alumina sample was 155 nm.