Abstract: The presence of fine polypropylene (PP) fiber with diameter ≤ 100 μm causes a fluctuation of the type I fracture strength in concrete. However, the mechanism of this phenomenon remains unknown, and the present mechanical models cannot describe the mechanical behavior of the fine PP fiber reinforced concrete (PFRC) due to the extremely expensive computational cost caused by the great number of fine fibers in matrix. To make up this gap, a mesoscopic discrete model is proposed by considering the improvement of water/cement ratio nearby PP fibers as the positive effect and the less mechanical contribution of fiber bridging force as the negative effect. The defining of the equivalent coefficient (r_f) of fiber diameter makes it possible to simulate the mechanical behavior of PFRC. The numerical results show that the equivalent coefficient (r_f) is recommended to be smaller than 10. The initial addition of PP fiber can increase the matrix strength, so that to slightly increase the PFRC type I fracture strength. The increase dosage of PP fiber causes the reduction of type I fracture strength due to the small strength contribution of fiber bridging force.