The aim of this study is to investigate the degradation process of boron filled polymers using fundamental and harmonic components of leakage current. The test procedure followed the ASTM D2303 standard. During the test, 50 Hz cycles of data taken from ground electrode were acquired at a sampling rate of 48.000 samples per second. The leakage current signal was processed by the Fast Fourier Transform (FFT) technique. The effect of boron on degradation process of polymers was studied between fundamental to 9th single harmonic components. From the experimental data which was taken from borax filled polyesters have higher harmonic components and lower fundamental leakage current than the pure polyester samples.