A method for improving EBG (electromagnetic bandgap) structures is provided. First, a multi-layer board having at least one EBG unit is provided. Then, a maximum input impedance of the EBG unit under a predetermined frequency band is measured, in which a frequency corresponding to the maximum input impedance is a resonance frequency, and a capacitance is determined based on the resonance frequency. Besides, a minimum input impedance of the EBG unit is measured, and a logarithmic value corresponding to the maximum input impedance and a logarithmic value corresponding to the minimum input impedance are obtained so as to determine a resistance. Finally, an electronic device having the capacitance and the resistance is coupled to the EBG unit in parallel.