A control circuit is provided for a microwave oven in which a crystal rectifier is responsive to variations in the level of the electromagnetic field within the oven cavity when items of different energy absorption or reflection characteristics are placed in the oven. The electric current developed by the crystal is employed to generate a ramp type electrical signal which is compared with a fixed reference voltage to control the termination of microwave power to the oven. Compensation for the nonlinear relationship between crystal current and the level of the electromagnetic field within the oven is provided. In addition, compensation for the initial temperature of the food item which is placed in the oven is provided. Provision is also made for physically shaping and/or partially shielding individual food items relative to a reference food item so that the same triggering level of the control circuit is effective to bring all of the food items to the desired condition at different time intervals depending upon the field strength level within the oven which is established when the food item is placed therein.