A method for treating, i.e. thawing, thawing and heating, or thawing and cooking, a frozen food (9) in a microwave oven (1) is provided comprising a microwave generator or magnetron (2), characterized in that the food (9) is interposed between the magnetron (2), initially operating at constant power, and an element (14) of microwave-sensitive material, the temperature (T) of the element (14) varying during the thawing of the food (9), the temperature (T) variation being monitored by a microprocessor circuit (21) which on the basis thereof evaluates the weight of the food (9). The microprocessor (21) then, on the basis of this evaluated weight and fed information regarding the type of food (9) and required type of treatment, determines the operating power of the magnetron (2) and the treatment duration.