The fatigue crack propagation behavior of as-extruded AZ31B magnesium alloy welded joint and heat affected zone have been studied. Compact tensions [C(T)] of welded joint [L-T(W)] and heat affected zone(welded joint is parallel [T-L(H)] and vertical [L-T(H)] to the extruded direction) were researched. Results indicate that the crack propagation direction is parallel to the extrusion direction for L-T(W); fatigue crack propagation rate is a rapid-slow process. T-L(H) is parallel to the notch direction and L-T(H) can be divided into two states (i.e., parallel to or angularly deflected towards the notch direction); crack propagation rate initially goes through a rapid course before it slows down. The fracture modes of crack tip remain a mixed-mode of transgranular and intergranular fractures. The fatigue fracture consists of quasi-cleavage and is a brittle fracture