The relationships between plastic zone, pores, fatigue crack growth rate (da/dN), and fracture mechanics parameters of metal injection molding (MIM) 316L stainless steel at various stress ratios (R= 0.1 and 0.4) are investigated in the present work. Using two-dimensional (2D) plane strain finite element analysis (FEA), the stress intensity factor and cyclic-plastic zone at crack tip are evaluated. The slow-growth da/dN at near-threshold regime is in brittle manner, and can be characterized by maximum stress intensity factor (Kmax). While, the mid-growth da/dN at Paris regime is in ductile manner, and can be characterized by stress intensity factor range (K). As a fracture mechanics parameter that combinesKmax and-K, theK* successfully characterizes the stress ratio effect on FCG rate at near-threshold and Paris regimes. The transition from slow-growth da/dN to mid-growth da/dN occurs when the cyclic-plastic zone at crack tip coalesces with pore. The appearances of fracture surfaces at near-threshold and Paris regimes are in good agreement with the FEA results. © Springer Science+Business Media B.V. 2012.