The dual active bridge (DAB) converter has been widely used as a dc transformer. However, little work has so far been undertaken to give an efficiency optimization method for DAB considering every single component. This paper gives an efficiency optimization method for DAB under single phase shift (SPS) control mode by designing the inductor and transformer. Some constraint conditions are analyzed in detail. The circuit level constraint conditions enable the converter to achieve the rated power and ZVS. The component level constraint conditions ensure that the core is not saturated, the window area is enough, and the temperature rise is acceptable. After the feasible region is ensured, the optimization problem for the efficiency is given and solved, obtaining the inductance and number of turns for the transformer. An experimental prototype is built, verifying the rationality of the constraint conditions and optimization. For the optimized result, the efficiency can rise to 98.9% at 7 kW.