The size effect of concrete structures is important for the design of concrete structures. In this paper, the stress distribution of a beam under three-point-bend is analyzed with finite element method under the assumptions that the shape, size and width of crack tip are constant and the length of the crack changes with the size of the structure proportionally. The results show, that the maximum tensile stress in the structure increase with the increase of the structure size, but with increase of the structure size, the size effect is reduced gradually and the size effect is less affected by shape of the crack tip.
