The gradation of rockfill affects creep behaviour, but indoor testing apparatus does not allow a field maximum particle size of the rockfill of 1000 mm. To investigate the creep behaviour of rockfill with different maximum particle sizes and establish a creep model for the rockfill material, oedometer creep tests for slate rockfill material with different maximum particle sizes were conducted to explore the effect of gradation on the creep behaviour of rockfill materials. The test results showed that the creep strain increases and the rate of increase decreases with the increase in vertical stress. The creep strain increases with the decrease in maximum particle size under all vertical stresses. Furthermore, a particle size ratio R_d was defined to establish the creep model. An empirical creep model was proposed to calculate the creep strain for rockfill with oversized particles that are not allowed by the experimental apparatus. Three groups of test data were used to determine the model parameters, and one group of test data was used to verify the predictive capacity of the model. The results showed that the model can satisfactorily predict the creep behaviour of slate rockfill with oversized particles.