To evaluate the condition of stator winding insulation in generators that have been operated for a long period of time, diagnostic tests were performed on the stator bars of a 500 MW, 22 kV generator under accelerated thermal and electrical aging procedures. The tests included measurements of AC current (${Delta}I$), dissipation factor ($tan{delta}$), partial discharge (PD) magnitude, and capacitance (C). In addition, the AC current test was performed on the stator winding of a 350 MW, 24 kV generator under operation to confirm insulation deterioration. The values of ${Delta}I$, ${Delta}tan{delta}$, and PD magnitude in one stator bar indicated serious insulation deterioration. In another stator bar, the ${Delta}I$ measurements showed that the insulation was in good condition, whereas the values of ${Delta}tan{delta}$ and PD magnitude indicated an incipient stage of insulation deterioration. Measurements of ${Delta}I$ and PD magnitude in all three phases (A, B, C) of the remaining generator stator windings showed that they were in good condition, although the ${Delta}tan{delta}$ measurements suggested that the condition of the insulation should be monitored carefully. Overall analysis of the results suggested that the generator stator windings were in good condition. The patterns of PD magnitude in all three phases (A, B, C) were attributed to internal discharge.