This paper presents the design of two flat transformers in LLC resonant converter. 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. In the LLC resonant converter, the switching loss is reduced by soft-switching. Therefore, LLC resonant converter can be operated at high switching frequency with high efficiency. However, transformer loss becomes problems at high switching frequency. As a result, flat transformers temperature becomes high in proportion to the transformer loss. Therefore, it is necessary to reduce the transformer temperature by analyzing the loss. In this paper, design for reduction transformer temperature is presented.