This paper presents the design of two flat transformers in low-profile LLC resonant converter for low transformer loss. The trend toward high power density, high efficiency, and low profile in power supplies has exposed a number of limitations in the use of magnetic component structures. Therefore, LLC resonant converter can be operated at high switching frequency with high efficiency, because the switching loss is reduced by soft-switching. However, flat transformer loss becomes problems at high switching frequency. As a result, flat transformers temperature becomes high. Therefore, it is necessary to reduce the transformer temperature by analyzing the loss. In this paper, design for low transformer loss is presented.