The size of metallic iron particles in coal-based reduced materials was measured by optical image analysis in three different reduced samples. The obtained data were analyzed by frequency and cumulative distribution. The results reveal that optical image analysis successfully measured the particle size of metallic iron in the reduced samples. The frequency distribution curves or cumulative passing percentage of metallic iron particles exhibit almost the same trend with respect to particle size. The quality of curve fitting indicates that the exponential decay function and Rosin-Rammler equation represent well experimental data on frequency distribution and cumulative passing percentage, respectively.