To provide a transformer assembly, in which the improvement of cooling efficiency of a transformer used for a microwave oven, a miniaturization by components reduction, weight decrease and production cost saving, an effective mounting structure are realized; to provide a method for manufacturing the transformer assembly; and to provide a microwave oven including the transformer assembly.

The transformer assembly includes the transformer for applying a voltage to a magnetron of the microwave oven, a container which contains the transformer and has a base plate jointed to the microwave oven and a cap portion jointed to the base plate, and a cooling substance contained in the container to cool the transformer. The method for manufacturing the transformer assembly is as follows: one end of a sidewall is joined on the base plate, the transformer is inserted in the sidewall, the transformer is mounted on the base plate, an upper plate is jointed to another end of the sidewall to complete the container, and an oil is injected into the container.