A simplified three-dimensional numerical model was presented to simulate the micro-crack nucleation and growth to some predefined dimension(approximately 0.38 mm) on the throat surface of a Ni-base powder metallurgy(PM) specimen.The numerical simulation of micro-crack initiation was based on the Tanaka-Mura micro-crack initiation models,where individual grains of the mesoscopic model were simulated using the Voronoi tessellation.Four improvements were made in the model.(1) Considering crack initiation along with 12 principal slip systems on octahedral slip planes of face centered cubic(FCC) crystal in three-dimensional(3-D) models.(2) Considering the effect of secondary phase precipitate due to impinging slip and dislocation pileup.(3) The Tanaka-Mura theory of fatigue-crack-initiation from notches was applied to simulate the crack initiation from another crack tip.(4) The coalescence of random initiated micro-cracks was simulated once they intersected with each other and a macro-crack was finally formed.The calculated results were in good agreement with the experimental data which verified the rationality of the simulation model.The applicability of the proposed model for treating fatigue-crack-initiation life in engineering structures was preliminarily achieved. A simplified three-dimensional numerical model was presented to simulate the micro-crack nucleation and growth to some predefined dimension (approximately 0.38 mm) on the throat surface of a Ni-base powder metallurgy (PM) specimen. Numerical simulation of micro-crack initiation was based on the Tanaka-Mura micro-crack initiation models, where individual grains of the mesoscopic model were simulated using the Voronoi tessellation. Flow improvements were made in the model. (1) Considering crack initiation along with 12 principal slip systems on octahedral slip planes of face centered cubic (FCC) crystal in three-dimensional (3-D) models. (2) Considering the effect of secondary phase precipitate due to impinging slip and dislocation pileup. crack-initiation from notches was applied to simulate the crack initiation from another crack tip. (4) The coalescence of random initiated micro-cracks was simulated once they intersected with each other and a macro-crack was finally formed. The calculated results were in good agreement with the experimental data which verified the rationality of the simulation model. The applicability of the proposed model for treating fatigue-crack-initiation life in engineering structures was preliminarily achieved.