Optimal city size in China is always a heated issue to be debated in China and the answer determines whether large cities or small cities should be give priority in the urbanization process. It is argued that the issue should be considered from both the perspective of energy and economic efficiency given the dual threats of global warming and fossil fuel crisis. This article extracts energy consumption data of 286 cities at the prefectural level and above in mainland China except Lasa, and analyzes the relationship between city size and urban energy efficiency. The relationship between city size and urban efficiency in economic, social and environmental aspects are investigated, and the findings reveal that large cities with 2 to 5 million population have the highest efficiency in all aspects.