The microstructure and mechanical properties of high performance sintered Fe-Ni alloy steels by using metal injection molding (MIM) process were investigated. The mixed elemental materials, iron and water-atomized Ni powders, were utilized in this study. The microstructure of the compacts have been consistently structured heterogeneously by molding and sintering the mixed elemental powders as same as our previous reports. In this study, the microstructural aspects of the compact were changed by the characteristics of Ni powder, such as particle size, shape, and distribution, which play important roles in the deformation behavior. Eventually, tempered compacts added 6 mass% fine Ni powder showed ultrahigh strength of more than 2 GPa (290 ksi) and elongation of higher than 8 %. In order to understand how the microstructure results these high mechanical properties, we developed 2D finite element modeling based on the spatial distribution obtained experimentally. The simulated results of the models were compared to experimentally obtained behavior, and showed good agreements.