This paper presents a method of dynamic voltage scaling (DVS) that tackles both switching and leakage power with combined V_dd/V_bs scaling and gives minimum average energy consumption exploiting the runtime distribution of software execution. We present a mathematical formulation of the DVS problem and an efficient numerical solution. Experimental results show that the presented method shows up to 44% further reduction in energy consumption compared with existing methods. Especially, when the leakage power consumption is significant, i.e. when temperature is high, the presented method is proven to be the most effective.