Abstract: In the process of determining particle size distribution in engineering fields, the sample capacity is one of determining factors to guarantee the accuracy and reliability of results. Combining with the theory of sampling reliability analysis, a theoretical method to determine the sample capacity is proposed and validated afterwards. It is found out that three indexes should be considered to obtain grading information of coarse aggregate: the total amount of particles, the degree of particle size difference, and the accuracy requirements. Besides, the proposed method may benefit engineering practice and specification perfection.