Microfibers (less than 100 µm in diameter) are commonly employed in structural applications to minimize early shrinkage cracking and lower pore pressure during fires. For any application, micro fiber-reinforced concrete (FRC) structural behavior and durability must be estimated using the mechanical constitutive law. Formulating a mechanical constitutive law for FRC presents several difficulties in terms of comprehending the physical principles and employing suitable numerical techniques. A novel model called “lattice discrete particle model for micro-FRC (LDPM-MicroF)” is presented to simulate the fracture behavior of micro-FRC. An equivalent fiber diameter coefficient has been defined to balance modeling accuracy and computational cost so that the LDPM-MicroF model can simulate the mechanical responses of engineered cementitious composites. The unimodal variation in tensile strength caused by the increase in microfiber dose is assessed and quantitatively reproduced by LDPM-MicroF predictions. This phenomenon is explained by a combination of mesoscopic mechanisms and the “near-field effect” of the fibers. A small number of microfibers can improve the strength of the matrix and thus slightly the tensile strength. However, when the dosage of microfibers exceeds a certain amount, the tensile strength decreases as the contribution of the fiber bridging force to the strength becomes lower than that of the replaced matrix. This research has provided new insights into the physical comprehension of the mechanical properties of micro-FRC, which has significant implications for the field of study.