In this paper, the power supply noise associated with package parasitics in an on-chip low-dropout (LDO) regulator is investigated. The on-chip LDO regulator with off-chip decoupling capacitors has power supply rail noise typically in the frequency range of few hundreds of MHz, which is related to the inductive package interconnects and the parasitic capacitance of the pass transistor. An equivalent circuit is proposed to model the power supply noise and understand the effect of inductive package interconnects. Based on the proposed equivalent circuit, the mitigation of the power supply noise from a package design perspective is discussed.