Abstract: In order to solve the problem of absent nondestructive test methods, the frequency domain spectroscopy (FDS) based on dielectric response theory had been used to test Vacuum Pressure Impregnating (VPI) less resin epoxy mica insulation of large generator stator bars. The effect of electrical aging of insulation dielectric properties was studied through comparing and analyzing the dielectric loss and dielectric constant curves in different ageing time and temperature. Result shows that the dielectric properties of epoxy-mica insulation decrease because of electrical aging, especially the epoxy resin. Molecular chain failure leads to the decrease of the bonding ability after the electrical ageing. More medium interfaces appear inside insulation material and the polar molecule activity improves at the same time. Interface polarization and turning-direction polarization are intensified. Dielectric loss and dielectric constant increase with electrical aging time.