To assess the degradation of stator winding insulation in high voltage motor, a total of forty-eight coils were manufactured. These coils are classified into five groups and there are twelve strands in each coil. Four coil groups are artificial defects such as shorted turn of strand, internal separation between conductor and insulation, large void within insulation and removal of semi-conductive tape. The other coil group is normal. Before ac breakdown test was performed, nondestructive test that included ac current, dissipation factor and partial discharge (PD) was done with five coil groups. This paper carried out by comparing the results between nondestructive test and ac breakdown test. When the values of ac current increase rate, tip-up dissipation factor and PD magnitude are large, the insulation is occurred the deterioration. AC breakdown voltage is also low in degraded insulation.