The intergranular fracture characteristics in nanocrystalline and ultra--fine polycrystalline metallic materials present intensive size effect and microstructure geometry effect. The conventional elastic--plastic constitutive theory is unable to describe these effects because it doesn't contain any length parameters to characterize the scale changing. Regarding this, a micro--structured model was proposed for the study on intergranular fracture of nanocrystalline and microcrystalline metals (mainly for the fcc metals). The hardening and size effects of material plastic deformation are described by the computational model based on the conventional theory of mechanism--based strain gradient plasticity (CMSG). A cohesive interface model was used to simulate the processes of grain--boundary sliding and separation, the initiation and propagation of intergranular cracks until the material fracture. The tensile experiment and stress--strain curves of nanocrystalline Ni were simulated by using the present model. Then the relation between macroscopic mechanical behaviors and intergranular crack's initiation and propagation in nanocrystalline Ni was investigated. Through the simulation to the experimental result in literature, the validity of the proposed model calculated nanocrystalline and ultra--fine polycrystalline mechanical properties was confirmed. At the same time, the simulation results show that the high strain gradient effects and severely plastic hardening of grain are induced by inhomogeneous plastic deformation, and the grain boundary induced deformation has a significant influence on the overall mechanical properties of nanocrystalline metals.