Fine and coarse aggregates play an important role in the fracture of concrete. However, quantitative information available on the effect of the coarse aggregate size on the fracture properties of concrete is still limited. In the present paper, the effect of coarse aggregate size (single grade of 5～10, 10～16, 16～20 and 20～25 mm) on stress-crack opening (σ-w) relation in normal and high strength concretes (compressive strength of 40 and 80 MPa, respectively)was studied. The investigation was based on three-point bending tests implemented by fictitious crack analysis. The result shows that coarse aggregate size and cement matrix strength significantly influence the shape of σ-w curve.For a given total aggregate content, in normal strength concrete, smaller size of aggregate leads to a high tensile strength and a sharp stress drop after the peak stress. The smaller the coarse aggregate, the steeper the σ-w curve.By contrast, in high strength concrete, the effect of aggregate size on σ-w relation almost vanishes. A similar σ-w relation is obtained for the concrete except for the case of 20～25 mm coarse aggregate size. The stress drop after the peak stress is more significant for high strength concrete than that for normal strength concrete. Meanwhile, the smaller the coarse aggregate size, the higher the flexural strength. Fracture energy and characteristic length increase with increasing coarse aggregate size in both normal and high strength concretes.