Dependence of within-die delay variations on power supply voltage (V_DD) is measured down to 0.4V. The V_DD dependence of the within-die delay variation of manual layout and irregular auto place and route (P&R) layout are compared for the first time. The measured relative delay (=sigma/average) variation difference between the manual layout and the P&R layout decreases from 1.56% to 0.07% with reducing V_DD from 1.2V to 0.4V, because the random delay variations due to the random transistor variations dominate total delay variations instead of the delay variations due to interconnect length variations at low V_DD.