Diagnostic tests were performed on two high voltage(HV) motor stator windings. These tests included the measurement of insulation resistance, polarization index, AC current, dissipation factor($tan{delta}$) and partial discharge(PD) magnitude. Surface contamination of HV motor stator windings has an effect on the AC current and $tan{delta}$. When the stator windings were finished cleaning and insulation reinforcement, the increase rate of AC current(${Delta}I$) and dissipation factor(${Delta}tan{delta}$) were very small compared to those before cleaning. However, the PD magnitude remained the same. These tests show that cleaning and insulation reinforcement of HV motor stator windings can reduce the insulation failure.