A microwave ablation apparatus 1 comprises: a source of microwave power 4； a microwave probe 20 comprising an antenna extending through a body 22； a layer of thermally insulating material 42 separating a thermocouple 40 from the body 22； a controller 2 which takes the thermocouple output voltage to determine a temperature value. A total amount of energy supplied to the antenna between starting an ablation and determining the temperature value forms a correction factor thats applied to the determined temperature value, so producing a corrected temperature value. The controller 2 controls microwave energy supplied by the source 4 to the antenna, so that the corrected temperature value reaches a predetermined target value. The correction value may also relate to: the location of the thermocouple 2 on the body 22； the time of applying the microwave energy； a temperature of a cooling fluid contained within the body 22. The corrected temperature value may be a predetermined multiple of the determined temperature value. The apparatus may comprise a plurality of thermocouples. The controller 2 may terminate the supply of microwave energy when the corrected temperature value reaches the predetermined target. Also disclosures suitability in determining an estimate of an ablation size.
