In highly inhomogeneous microstructures with a wide range of grain sizes, the problem arises of defining an effective average grain size. An original method for the estimation of effective average grain size and a parameter describing the homogeneity of microstructure are presented. The coefficients of homogeneity for equiaxed and columnar microstructures are calculated. The effective average grain size is calculated as a weighted sum of the average grain sizes of homogeneous grain groups. (c) 2005 Springer Science + Business Media, Inc.