The Dynamic Voltage Scaling (DVS) technique has been widely studied for energy-constrained real-time systems; however, its application to control systems has not been studied in a variety of aspects. This article presents a novel method to simultaneously schedule processor voltage and control-task periods online, considering energy-efficiency of control systems as a whole. Anew performance index is proposed, which contains both control performance and processor energy terms. Then, an online scheduler assigning processor voltage and control-task periods that maximizes the performance index is proposed. The performance of the proposed scheduler under varying control workload is verified using MATLAB simulations and experiments on an actual DVS hardware platform.