Abstract: During the investment casting of aeroengine blades, the swelling and the fracture phenomena appear frequently in the B-type thermocouple (PtRh30-PtRh6) for melt temperature measurement, which results in the scrapping of castings. The chemical composition and microstructure of the B-type thermocouple with swellings and fractures, as well as the ceramic insulator and sheath, were detected by glow discharge mass spectrometry and scanning electron microscopy. At the same time, the thermal potential stability related to the microstructure of the B-type thermocouple after thermal exposure at 1500 ℃/100h was analyzed according to national standards. All the investigations were to obtain the fracture reason and mechanism of thermocouple. The results showed that the main reason for the fracture of the type B thermocouple was the presence of the phased with Al and Si rich contents in the thermocouple wire. Aside from that, it was found that the microstructure stability of the B-type thermocouple was poor, and there were significant changes in the surface structure and temperature measurement accuracy after thermal exposure, which seriously affect its service life and temperature measurement accuracy. Finally, based on the above results, the suggestions including the controlling the composition of raw materials and the microstructure stability for B-type thermocouples are proposed, which could reduce the abnormal fracture trouble of the thermocouple.