Abstract: After testing, a linear display on tenon tooth of the first stage working turbine blade was found during fluorescence detection. Macro and micro observation, BEI and EDS were carried out to analyze the "crack" and "fracture" surface. Besides, the microstructure of the blade was examined. The results show that the linear display was because there were Hf-rich oxide inclusions, leading to cracking during testing. In the process of smelting, plate HfO₂ inclusions formed due to shell mold reaction. Some measures can be taken to prevent such oxide inclusions, such as controlling the content of Hf and the vacuum degree, strengthening slag cleaning and stirring during smelting.