Abstract: The lubricating oil system of an aero-engine alarmed during flying. Crack was found between the oil supply pipe and the cover plate after inspec-tion. The crack propagated along the circumference, accounting for about 4/5 of the whole circumference. The cause of crack was analyzed by fracture obser-vation, metallographic analysis, finite element analysis, and assembly stress and vibration stress testing. The results show that the crack of the oil pipe is high cycle fatigue crack. The crack initiated from the outer surface at the corner of the brazed weld. The assembly stress at the crack position is 280 MPa, and the vibration stress is 21 MPa.The larger vibration stress and assembly stress are the main causes for the crack. Clamp should be designed and the alignment should be improved to prevent such a failure.