As a part of evaluating the Friction Stir Welding (FSW) performance of DH-36 steel, the fracture surfaces of the friction stir welded tensile specimens were observed after fracture under both a monotonic axial and a cyclic loading. Monotonically loaded welded specimens always fracture on the parent material in a typical ductile behavior. Fatigue loading creates stress concentrators in the material surface by the stir tool shoulder. Surface micro-cracks are nucleated in the vicinity of the stress concentrators and propagate in different patterns depending on the plate thickness causing the material failure. In this paper, microstructure of the weld region, microhardness variation and state of stress on the nucleated cracks tip are studied and correlated to the observed fracture patterns.