Abstract: There is a certain difference in compact density of rockfill materials between the real gradation and corresponding scaling gradation. In order to describe the difference quantitatively, the scaling effect can be divided into two parts, changes of the maximum particle size and the gradation structure. With the mentality that the continuous gradation is combined with the similar gradation method, many gradations of rockfill materials are designed, and their maximum particle sizes are 60, 40, 20 mm. The surface vibration compaction tests are conducted to study the effects of the maximum particle size and gradation structure on the compact density of rockfill materials respectively. The effects of the maximum particle size and gradation structure on the compact density can be described quantitatively by using the proposed function form. The prediction model for the compact density of rockfill materials is proposed, which considers the effects of the maximum particle size and gradation structure, the applicability of different rockfill materials is discussed, and the accuracy of this model is verified through a number of laboratory compaction test data. In addition, a way for quantitatively studying the scaling effect of rockfill materials is proposed.