Cracks in the blades’ root of turbo rotor made from high temperature alloy were detected by fluorescence examination after several engine trial runs. The results indicate that the cracks are located in the thin-walled area in the blade， and the cracking mode is high temperature fatigue cracking， which can be classified into low-cycle fatigue and life-relevant failure. In the trail run process， the stress concentration location is exposed to the effect of high temperature and alternating stress. Therefore， selective oxidation in the grain boundaries happen and oxide products grow in a wedge-like shape along the grain boundaries.By the mutual influence of both high temperature oxidation and alternating stress， the stress concentration location in the blades’ root also experiences creep and slippage.With the effect of creeping damage and intergranular oxidation， the weakened grain boundaries start to crack and cracks expand along the grain boundaries， forming the fatigueorigin. At last， high temperature fatigue cracks further expand in the subsequent process.