The characteristic parameters of the stator winding insulations in the inverter-fed traction motors with various service years, including the insulation resistance, dissipation factor, partial discharge inception voltage (PDIV) and residual breakdown voltage, are measured sequentially to analyze the insulation deterioration. The insulation materials from different sites of the insulation along the windings are sampled and analyzed with thermogravimetric analysis (TGA). The measured dielectric results show that the characteristic parameters from the stator windings decrease slightly in comparison with those of new motors. The aging degree of the insulation near the outlet slot is higher than that in the slot. Besides, the TGA results from the sampled insulation materials indicate that the stator winding insulation in the inverter-fed traction motor with a service period up to ten years shows slight deterioration. Generally, more serious deterioration in the insulation tends to appear at the bottom side of the motor coils.