Though the test level has improved highly, the original grading curve of coarse materials should be reduced in some scale in dynamic triaxial test. This method must affect the difference between the test results and the real results, which is called scale effect. In this paper, the scale effect was studied by using particle flow code (PFC) based on dynamic triaxial tests. The relationship between microscopic shear modulus and the maximum particle diameter can be simulated by hyperbolic function. The results show that the dynamic shear modulus and damping ratio are increased with the sample size increase. The normalized dynamic shear modulus vs. normalized dynamic shear strains of different size samples are located in a narrow space which can be simulated by modified Hardin-Drnevich model formula. The parameters of maximum shear modulus and damping ratio of different sample size can be simulated by power functions.