In studies of finite-time thermodynamics, most performance analyses concern the maximum power output and the corresponding efficiency for heat engines. In this paper, instead of just maximizing power for a cycle, the power density (the ratio of the power to the maximum specific volume in the cycle) is maximized for an Atkinson engine. The results showed that the efficiency at maximum power density is always greater than that at maximum power, and the design parameters at maximum power density lead to smaller and more efficient Atkinson engines with larger pressure ratios.