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In this paper, an image processing technique is introduced to measure the grain size and their distributions from the SEM image of copper oxide (CuO) and titanium dioxide (TiO2) doped sintered alumina ceramics accurately. The noise present in SEM image is removed by applying low pass Gaussian filter followed by suppression of regional minima over a threshold. The clarity of individual grains and grain boundaries have been done by applying Watershed transform to this preprocessed SEM image. Morphological operations like dilation and erosion are used to make the grain‐boundary edges clear and continuous. The individual grain size in µm scale is measured from the pixel length of the rectangular bounding box drawn around the segmented grain. The normal Gaussian type distribution of grain size is observed in both CuO‐ and TiO2‐doped grains in SEM image. The average grain size of CuO‐doped alumina grains (2.24 µm) is very close to G50 value (2.17 µm), but G50 value of TiO2‐doped grains (8.59 µm) is slightly higher than its average grain size (7.96 µm). The proposed algorithm is compared with linear intercept method and the grain sizes obtained are very close to each other. [ABSTRACT FROM AUTHOR]