Abstract: A microwave oven having a fan motor rotation control function comprising an absolute humidity sensor for sensing the humidity inside the microwave oven, a fan motor for discharging water vapor inside the microwave oven externally, a microprocessor and a keyboard. The improved microwave oven further comprises a phototriac connected between said fan motor and a supplying line of AC power source for controlling AC power supplied to the fan motor in accordance with a plurality of control signals from the microprocessor and a zero crossing signal generating circuit for detecting a zero point from AC power voltage during a constant cycle to generate a predetermined width of pulse signal at the zero point and for applying the predetermined width of pulse signal the microprocessor.