Traditionally, fatigue life, damage and FOS have been computed by applying fatigue codes (e.g., nCode, FESafe, OS Fatigue) to the stress vs time results from a transient finite element analysis. The transient analysis requires a load vs. time history which can be obtained from a multi-body dynamics analysis. With the advent of flex-bodies, it is now possible to extract the necessary stress vs time history directly from the MBD results, thus eliminating the need to obtain the load vs time history or to run the transient analysis. This paper details the modal synthesis process, proceeding from MotionSolve MBD analysis directly to the OptiStruct fatigue analysis.