It was found through the experimental approach for Al alloys that long freezing range alloys (LFRA) have a tendency to show the dispersed porosity distribution over the casting. In the present numerical analysis, the prediction parameters used in short freezing range alloys (SFRA), such as t_f, G and G/√R are not applicable to describe the porosity formation for LFRA. Thus fluid critical solid fraction, fs-critical was utilized as a prediction parameter to evaluate the porosity formation for LFRA quantitatively. It was shown that the calculated results using fs-critical were in good agreement with those of test castings with the cylindrical geometry. It was concluded that fs-critical successfully predicts the detailed porosity formation behavior for LFRA in correlation with the concept of potential shrinkage. (Received February 5, 1999)