The relationship between the pore size distribution and the separation characteristics of metallic filter media is investigated and a method for estimating the partial separation efficiency is proposed. First, the pore size distribution of metallic filter media is measured via the bubble point method. The broadness of the distribution depends on the type of filter media, but the distribution is found to be in accordance with a normal distribution. In addition, pore size obtained by the bubble point method and separation particle size in actual filtering operations are not consistent due to the variety of pore shapes. Thus, the correlation between the pore size and the separation particle size is determined for individual types of filter media by performing a filtration test. Then, a method for estimating partial separation efficiency based on the sieving theory is proposed and its validity is confirmed. This method can estimate the partial separation efficiency of metallic filter media via the non-destructive measurement of the pore size distribution, and so gives valuable information when selecting a suitable filter medium for filtering operations.