In order to evaluate the grade of metal bonded synthetic diamond, it is useful to examine the shape of diamond grains using scanning a electron microscope. Distribution of the largest and smallest diameters of the grain obey normal distribution as a first approximation. The grains comprise a mixture of cubes and octahedra, and it is observed that {100} and {111} planes construct the cubic and octahedron planes, respectively. Then, we defined various kinds of shape coefficient that denotes the magnitude of deviation from a sphere, which showed good correlation with the strength of the diamond. Therefore, the magnitude of strength of diamond can be examined using the shape coefficients defined here.