A calibration method of insulating washer in a circuit board is provided, which includes steps of (a) establishing an equivalent circuit model corresponding to a metal via; (b) depicting an electric characteristic curve corresponding to the model; (c) calculating a capacitance and an inductance equation corresponding to the curve; (d) substituting parameter values of the equivalent circuit model, capacitance equation, and inductance equation into a characteristic impedance expression for being calculated, so as to obtain a first radius length value of an insulating washer for the metal via; (e) making a temporary insulating washer by employing the value; (f) reestablishing an equivalent circuit model corresponding to the metal via according to the temporary insulating washer; repeating Steps (b), (c), (d), (e), and (f); and when the first radius length value is converged into a fixed value, determining the fixed value as an optimum value for designing the insulating washer.