The service-life of concrete structures depends on the delayed strains that appear due to creep phenomenon. Few are the studies that treated the effect of the dimensions of concrete specimens on the amplitude and the kinetics of creep and the results show many contradictions. Thus, to design reliable civil engineering structures, the knowledge of the behaviour of concrete under a sustained load including size effect is necessary and performing calculations are needed. In this paper, the physical mechanisms behind the size effect on creep rate are evaluated at the mesoscopic scale. The material volume is modeled, by a Digital Concrete model which takes into account the microstructure heterogeneities and the “real” aggregate size of concrete. Calculations are performed in 2D by considering a viscoelastic damage behaviour law for the matrix and an elastic behavior for aggregates. The numerical results show that size effect is well reproduced by the meso-scale approach. The stresses under a sustained load are induced by strain incompatibilities between the components at the mesoscale. Accordingly, the evolution of the microcracked zone with the size of the bending specimens can be related to the creep rate.Read less <