Provided is an air flow passage of a microwave oven including: a cavity for accommodating foods therein; an electric component chamber disposed at a predetermined portion of the cavity; a suction hole formed at one sided portion of the cavity such that the cavity communicates with the electric component chamber; an exhaust hole formed at the other sided portion of the cavity, through which air sucked through the suction hole is exhausted; an exhaust guide covering an outer wall of the cavity where the exhaust hole is formed, for guiding air exhausted through the exhaust hole to an outside of the microwave oven; and a back plate having an exhaust passage hole communicating with one end of the exhaust guide, through which air is exhausted. According to the present invention, hot air of an inside of the microwave oven can be smoothly exhausted. In particular, since the exhausted air does not return to the electric component chamber, cooling efficiency can be enhanced.