Turbine blades crack problem is widespread. Combined with the characteristics of hydropower, analyzes the causes of cracks in the turbine, materials, design and manufacturing, operation and so on. Plant No. 6 in Little Bay machine as an example, were detected by macroscopic, chemical composition analysis, crack cross-section analysis, microstructure examination, crack source electron microscopy and spectroscopy to analyze the crack, the final cause of the crack is leaves the local area resonance, the resonance of the blade in the crater formed at the source of fatigue and fatigue development along at the source, the formation of fatigue cracks.