OBJECTIVES: The aim of this study was to investigate polarity effects of electron beams with different energies. METHODS: Energy dependence of polarity error was investigated at dmax and SSD=95 for 6, 8, 10, 12, 15 and 18 MeV electron beams from Phillips SL-25 LINAC. Field size dependence of polarity effect was investigated at dmax and SSD=95 cm using 6x6, 10x10, 14x14 and 20x20 cm2 standard electron cones for 6, 8, 10, 12, 15 and 18 MeV electron beams from Phillips SL-25 LINAC. RESULTS: Polarity effect decreased as electron beam energy increased. While polarity error was 0.52% for high energies, it increased up to 4.5% for low energies. Moreover, polarity error increased as field size increased for the same electron energy. CONCLUSION: In conclusion, especially for low energies and wide field sizes, polarity error is measured and used in absolute dose calculations for parallel plate chambers.