The fatigue crack propagation rate of as-extruded AZ31B magnesium alloy was studied. Compact tension [C(T)] of the notch direction parallel (T-L), vertical (L-T), and inclined at 45o to the extrusion direction was investigated. The experimental results indicate that the crack propagation direction is parallel to the extrusion direction for T-L and L-T specimens, whereas the specimen inclined at 45o has an angular deflection of 9° to 11° toward the extrusion direction. The T-L specimen has the fastest fatigue crack propagation rate, and the L-T specimen has the slowest rate, the fatigue crack propagation rate of the specimen inclined at 45o is between the two directions. The crack tip propagates by both transgranular and intergranular fractures. Fatigue fractures consist of cleavage plane or quasi-cleavage and are brittle fractures. The fatigue striation occurs for specimens inclined at 45o and its size is 3-15 μm. The fatigue crack propagation rate of as-extruded AZ31B magnesium alloy was studied. Compact tension [C (T)] of the notch direction parallel (TL), vertical (LT), and inclined at 45o to the extrusion direction was investigated. results indicate that the crack propagation direction is parallel to the extrusion direction for TL and LT specimens, whereas the specimen inclined at 45o has an angular deflection of 9 ° to 11 ° toward the extrusion direction. , and the LT specimen has the slowest rate, the fatigue crack propagation rate of the specimen inclined at 45o is between the two directions. The crack tip propagates by both transgranular and intergranular fractures. Fatigue fractures consist of cleavage plane or quasi-cleavage and are brittle fractures. The fatigue striation occurs for specimens inclined at 45 ° and its size is 3-15 μm.