A new kind of power analysis is conducted on a reversible Joule-Brayton cycle. Although many performance analyses have been carried out resulting in famous efficiences (Carnot, Curzon-Ahlbom), most do not consider the sizes of the engines. In the studies of Curzon and Ahlbom and others, researchers utilized the thermal efficiency at maximum power as an efficiency standard for practical heat engines. In this paper, instead of just maximizing power for certain cycle parameters, the power density defined as the ratio of power to the maximum specific volume in the cycle, is maximized. Therefore the effects of the engine sizes were included in the analysis. The result showed a new type of efficiency at the maximum power density which is always greater than that at the maximum power (Curzon-Ahlborn efficiency). Evaluations show that design parameters at the maximum power density lead to smaller and more efficient Joule-Brayton engines.