Abstract: An early crack was found in the middle of a fan blade in an aero-engine. The cracking mode and cause were analyzed by appearance observation, macro and micro observation on fracture surface, surface inspection and material analysis. The results indicate that the failure mode of the fan blade is high cycle fatigue cracking. The fatigue source was located at the sub-surface of the back side in the middle of the fan blade. The main causes for the cracking were the non-uniform distribution of metallurgical structure in the middle of the blade and lots of lath-shaped primary α phase at the fatigue source region. The fatigue resistance of the fan blade decreased significantly. The improvement measures for avoiding formation of lath-shaped primary α phase are to control the forging temperature and to ensure the deformation degree of the raw material.