A general method to quantify the inclusion dispersion of mixture microstructures has been developed. The dispersion quantity, D, is defined as the probability of inclusion particle free-path spacing falling into a certain range of the mean spacing μ, according to the particle spacing data frequency distribution. Two quantities, D0.1 and D0.2, are proposed, which are the probabilities of the particle free-path spacing falling into the ranges of μ ± 0.1 μ and μ ± 0.2 μ, respectively. Both normal and lognormal distributions are discussed, and in both cases, the quantities D0.1 and D0.2 are specified as monotonous increasing functions of μ/σ, where μ and sigma; are the mean particle free-path spacing and standard deviation, respectively. Examples of composite are presented to quantify the dispersion of foreign reinforcements based on the proposed method.