Abstract: In order to obtain the impact of coarse aggregate size on the size effect of concrete flexural strength, the four-point bending tests were performed for 27 group specimens. The flexural strengths of specimens with the dimensions including 100mm×100mm×400mm, 150mm×150mm×550mm, 200mm×200mm×700mm and the maximum aggregate size including 10, 20 and 30mm respectively were obtained. The test data were analyzed by Bazant size effect law, and the index-type size effect law was proposed. The results show that the strength of the specimens decreases with the coarse aggregate size increasing under the same aggregate content, water cement ratio and specimen size. The size effect of concrete strength is apparent for the specimens of three aggregate sizes, and the smaller the specimens size,the more obvious the size effect is. With the specimen size increasing, the strength of specimens with 30mm aggregate size decreases more gently than others. The Bazant size effect law fits well the test data. However the law is not suitable for the whole specimen scale. Therefore the index-type size effect law was proposed, which could be used to predict the size effect of the flexural strength of concrete in the whole specimen scale.