Abstract: This study has carried out the three-point bending test with the digital image correlation technology（DIC） to explore the mechanism of size effect on the tensile properties of clay, and the tensile crack failure process of compacted clay beam under the condition of geometric similarity and non-geometric similarity（the change of sample height h, thickness b） is analyzed from macro and micro perspective.The results showed that: The increasing of the size of geometrically similar and non-geometrically similar specimens prompted the increasing of the peak load of the specimens. The tensile strength σ_t of soil is significantly affected by the size effect and σ_t of geometrically similar soil beam linear decreases with the increase of sample size. When only increasing the soil beam h, σ_t increases at first and then decreases. When the soil beam b is changed, σ_t decreases first and then increases with the increase of b, in which the change of b has less influence on σ_t. By introducing the meso-evaluation index: Local strain degree εa, it is concluded that the εa of geometrically similar specimens increased with the increase of size, while the ε_a of non-geometrically similar samples increased at first and then decreased with the increasing of h or b.