A test study has been carried out for the microstructure aging of high-temperature surperheater and medium -temperature reheater tubes made from T91 steel for two utility boilers. Results show that the characteristics of microsturcture aging of the materials are difficult to be observed under an optical microscope; but the fine structure of microstructure aging can be obviously observed under a transmission electron microscope(TEM). The microstructure aging of medium-temperature reheater tubes during short-term operation is more serious than that of high-temperature superheater tubes during long-term operation.