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We pay attention to an insulation diagnosis technique of on-line hydrogenerator stator windings. On-line partial discharge monitoring and diagnosis of the insulation of hydrogenerator machines can be expected to reduce their maintenance costs. Therefore, we aim at development of the hybrid diagnostic method that ultrasonic wave signal occurring by partial discharge and leakage current flowing through neutral grounding cable of hydrogenerator. We set up many AE (acoustic emission) sensors to stator core and set up a CT (current transformer) sensor at neutral grounding cable of hydrogenerator. We measured AE signal and neutral grounding cable leakage current of hydrogenerator on stream. Furthermore, we set up measurement equipment in a hydrogenerator wing channel and measured AE signal and leakage current flowing at neutral grounding cable by partial discharge for a long term. In this paper, we have measured the relationship of on-line hydrogenerator out put and AE signal and leakage current flowing at neutral grounding cable. And, we have measured the AE signal and leakage current on the dielectric loss tangent test and partial discharge pulse at off-line test.