High frequency currents flowing from power terminals of an IC/LSI onto power/ground planes of a digital printed circuit board can generate a great amount of electromagnetic noise. A power current model of an IC/LSI for EMI simulation was proposed in our previous studies. This report describes the mechanism and quantitative effect of reducing high frequency currnts on power/ground terminals by embedding a decoupling inductor and/or a bypass capacitor in an IC/LSI package. Furthermore, the above internal decoupling effect on suppressing EMI radiated from the board is verfied in both the simulations and measurements.