81 concrete prism specimens under axial compression were tested to invesgigate the size effect on the axial load stength. Three different kinds of specimens with the dimension 100×100×300mm, 150×150×300mm, 200×200×400mm were tested. The parameters including compressive strength of concrete and aggregate composition are taken into consideration. Three different strength grades of concrete and three different aggregate composition are included in those specimens. The test method are undertaken according to ASTM C 39/C 39M-2005. Based on the test results, a new size effect law for different kinds of concrete in prismatical compressive strength is suggested and those relative parameters on the size effect are discussed.