Abstract: A microwave oven incorporating therein an air cooling system comprises an outer casing containing a cooking chamber and a compartment. The compartment is provided with an air intake opening. A magnetron and a high voltage transformer are positioned at upper and lower sides within the compartment, respectively. A cooling fan is disposed at a position near the high voltage transformer and forces cooling air around the microwave oven to enter the compartment through the air intake opening and to leave the compartment along a plurality of airflow routes. A screen plate is disposed between the high voltage transformer and the cooling fan and directs a part of the cooling air from the cooling fan to the magnetron so that the magnetron is directly cooled by the cooling air.