Abstract: The size effect on the deformation parameters directly affects the deformation prediction of earth-rockfill dams and the deformation coordination of their seepage body. It is a key technical problem for high earth-rockfill dams. Based on the existing researches, the stress-strain relationship considering the size effect is demonstrated, and the particle size correlations for parameters of the Duncan-Chang model are deduced. The results show that the stress-strain relationship of samples with larger particles for crushable rockfill materials can be predicted based on the size-related strength of single particle and stress-strain curves of samples with smaller particles. In the Duncan-Chang model, the parameters with obvious size effect are φ₀, K and K_b. The remaining parameters, such as R_f, n and m_b, are invariant parameters or have no significant size effect. The rationality of this rule is further verified by comparing the predicted Duncan-Chang model parameters with the test results for samples with different particle sizes.