This paper demonstrates the utilization of gallium nitride (GaN) devices to achieve an ultrahigh efficiency in an isolated dc-dc converter. This paper presents the design and implementation of high-efficiency magnetics necessary to realize such an ultrahigh efficiency converter. Synchronous rectification is also implemented to further improve the efficiency. Compared to an equivalent silicon MOSFET, GaN devices have low output charge, very low gate drive losses, and zero reverse recovery losses, this makes GaN devices more suitable for power converters to achieve high efficiency. A complete analytical loss modeling for an isolated full bridge dc-dc converter is discussed in this paper. Furthermore, the experimental demonstration of ultrahigh efficiency in a 2.4-kW isolated GaN converter is presented. Ultrahigh conversion efficiency allows the realization of a very compact dc-dc converter with a very small or no heat sink. The prototype converter has a power density of 7 kW/L and the measured efficiency is above 98.5% over a wide range of output power. The maximum measured efficiency of the converter is 98.8% at 50% of full load.