Abstract: With the development of dam construction towards higher dams and larger storage capacity, the dam materials are also developing towards larger particle sizes. However, due to the limitations in conditions and scale, the soil tests need to be conducted in a scaled form, which has led to the issues related to size effects. In this study, a size effect model for the deformation characteristics of dam soil and stone materials is derived and established taking into account the factors such as particle breakage, lateral deformation, dry density and load level. A series of compression experiments are conducted on two typical types of dam building materials, rockfill and gravel with sample sizes ranging from 120 to 1000 mm, revealing the correlation between the size effects of diameter to diameter ratio and crushing rate. The research results indicate that the particle breakage rate decreases with the increase of the diameter to diameter ratio, the compression modulus decreases with the increase of the particle breakage rate, the deformation modulus of the rockfill materials increases with the increase of the diameter to diameter ratio, and the compression modulus of the gravel materials decreases with the increase of the diameter to diameter ratio. The experimental results are consistent with the the calculated ones by the model, proving that the proposed model is of certain applicability and rationality.