Near-threshold voltage operation is a well-known solution for drastically improving the energy efficiency of microprocessors fabricated with the latest process technologies. However, it is not well studied how the optimal gate size of standard cells changes when the supply voltage of the microprocessors gets closer to the threshold voltage. This paper first shows an experimental observation that the optimal gate size for nearthreshold voltage which is 0.6V in this work is far from the optimal gate size for the nominal supply voltage which is 1.2V in our target process technology. Based on this fact, the paper next presents our cell optimization flow which finds the optimal gate sizes of individual standard cells operating at the near-threshold voltage. The experimental results show that, when operating at the 0.6V condition, the energy consumptions of several benchmark circuits synthesized with our standard cells optimized for the 0.6V condition can be reduced by 31% at the best case and by 23% on average compared with those of the same circuits synthesized with the cells optimized for the nominal supply voltage.