Electromagnetic radiation is always headachy issue in developing various electronic systems. Because electro-magnetic radiation and interference (EMI) is very sensitive to signal routing and physical layout of power distribution network (PDN) in package and board as well as chip design. Therefore, chip-package co-design is becoming important by taking into account the total PDN impedance seen from the chip. Especially, parallel resonance peaks in the PDN due to the chip-package interaction induces the unwanted power supply fluctuation, and results in serious EMI troubles. In this paper, three test chips were designed with different on-chip PDN properties. These three chips were intended to have typical characteristics; oscillatory region and damped condition. Then, the effects of the total PDN impedance on EMI have been studied. In particular, anti-resonance peak was properly suppressed by establishing critical damping condition in chip-package-board co-design. EMI has been proved to be dramatically reduced by proper combination of on-chip capacitance, on-chip resistance and package inductance.