Vibration response analysis is applied to a classification of cast iron products. We have studied the correlation between the mass and the natural frequency of the products. The test pieces used here are some types of water pump impellers for automobiles. It is difficult to manually classify many kinds of impellers which have similar configurations and dimensions. We have found that the frequency characteristic of test pieces, which depends on the rigidity of a disk part, can be used for the classification. As a result, we have made a characteristic map on the mass-natural frequency plane of the products, which facilitates classification.