An interfacial resistance measuring method was proposed to evaluate the quality of diffusion bonded joints. To study the feasibility of the method, a finite element (FE) model with periodically distributed interfacial microdefects was presented. The influences of bonded area fraction, width and length of microdefects on increments of resistance were analyzed by applying the electromagnetic module in ANSYS software. Results indicated that the evolution of resistance increment along with the interfacial bonded area fraction could be described by a hyperbolic function. Based on the FE results, a modified theoretical solution was developed referred to Lodge’s work to accommodate the size effects of interfacial microdefect and thus could lead to a more precise quality evaluation for diffusion bonded joints.