This dissertation studies an accident resulting from the breakdown of an MVR impeller blade. Visual, stereoscopic and, SEM examinations have been carried out to find out the causes of the MVR blade defects. The results show that failure of the MVR blade was due to material casting defects. An initial crack started from casting defects where stress intensified and then, a fatigue crack progressed to the critical length of this crack. In addition to these defects, overstressing caused by overspeed accelerated the catastrophic failure of the blade.