The establishment of Condition Based Maintenance for hydrogenerators is very important for monitoring the conditions of stator windings insulation. On-line partial discharge (PD) testing has been used to provide useful information for monitoring the integrity and diagnosing problems in hydrogenerator stator winding insulation. The on-line insulation diagnosis technology can assess thermal, electric, ambient, and mechanical stresses received by the hydrogenerator stator windings. We have already indicated in a previous study that on-line PD measurements using acoustic emission sensor (AE) techniques can be useful for assessing the conditions of stator winding insulation. In this paper, the relationship between AE signals and maximum discharge magnitude was investigated in a non-destructive test for hydrogenerators using off-line measurements. AE signals were investigated by the use of multiple AE sensors, which are produced from an internal discharge for two hydrogenerator stator windings in 11kV and 13.2kV classes, in addition to the load and temperature, and humidity measurements taken on-line in twenty-month and three-month long studies respectively.