This paper reports simulation results on simultaneous switching noise and radiated emission due to the existence of on-chip decoupling capacitor. Signal overshoots/undershoots was observed for the case of on-chip decoupling capacitor, while signal degradation was observed for the case of without on-chip decoupling capacitor. The behavior of ground bounce for the case of on-chip capacitor was different from the case of without on-chip capacitor. The current flow at the power and ground terminals was used as excitation source of FDTD model with the power/ ground plane. The simulation results were compared with the measurement data.