Field test is performed in diverse environments, in which temperature varies across a wide range. As temperature affects a circuit delay greatly, accurate temperature monitors are required. They should be placed at various locations on a chip including hot spots. This paper proposes a flexible ring-oscillator-based monitor that accurately measures voltage as well as temperature at the same time. The measurement accuracy was confirmed by circuit simulation for 180 nm, 90 nm and 45 nm technologies. An experiment using test chips with 180 nm technology shows its feasibility.