Abstract: A simple method is presented for determining the material parameters of concrete K_IC and f_t using the small specimens in laboratory, in which the maximum aggregate of the concrete d_max=10 mm, the specimens depth W=60 mm, 80 mm, 100 mm, 140 mm and 160 mm, and the specimens width B=40 mm. Compared to existing size effect models, only linked to specimen size W, the concept "relative size W/d_max" is proposed. The roles of aggregate size on fracture of finite size specimens were analyzed, and the maximum aggregate d_max and discrete number β are included in the analytic solution expression of the improved Boundary Effect Model, named discrete particle fracture model. The different combination conditions of tested specimens and different fictitious crack growth at peak loading are adopted to determine the influence law of K_IC and f_t. The whole fracture failure curves of the concrete material are obtained. The theoretical minimum concrete specimen size meeting the condition of linear elastic fracture mechanics is obtained. And the peak loads of every specimens are predicted successfully based on the determined material parameters.