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A new diagnostic method, namely, velocity measurement of the ultrasonic longitudinal waves launched into and propagated in stator insulation, is put forward to assess the aging extent of stator insulation in this paper. The ultrasonic velocity was measured statistically as the minimal velocity (Vi,,) and the average velocity (Vav) on stator bars that had already been in service for 0-year, 18-year and 23-year periods respectively. The results show that Vmin decreases rapidly, and Vav decreases slightly with the insulation service time. Vmin and Vav measured on the stator bars in different stator slots of one generator vary, indicating the different aging extent of insulation due to the varied service conditions from bar to bar. Vmin and Vav are useful parameters to assess the aging extent of stator insulation of large generators.