A wave guide system of a microwave oven is provided, comprising a cavity containing a food to be cooked and having a pair of microwave feed openings formed in one wall thereof; a magnetron having an antenna and positioned between the microwave feed openings in spaced apart relation to the wall having the feed openings, to generate microwaves having a wavelength of .lambda..sub.g ; and a waveguide provided to cover the feed openings, support thereon the magnetron and guide the microwave through the feed openings into the cavity and having a short circuited surface which is spaced apart from the antenna by a distance of .lambda..sub.g /4 and parallel to the antenna. With this arrangement, the microwave generated in the magnetron produce standing waves in the waveguide, and then are emitted into the cavity through the microwave feed openings of the cavity, thereby uniformly heating a food in the cavity.