Percutaneous energy-based ablation has been used for the treatment of many tumour types. The advent of thermoablative technology for the treatment of small size liver tumour introduced several advantages over surgical resection. The technology involves the creation of an electromagnetic field through an intratumourally placed antenna. A process known as dielectric heating elevates the temperature of the tissue immediately surrounding the slot of the antenna where most of the coagulative necrosis occurs. Attenuation of the electromagnetic field as it travels through the tissue leads to decrease in the rate of tissue necrosis creating a zone of sub-lethal hyperthermia, and then a zone of tissue unaffected by ablation. The temperature spreads across the tissue to destroy the cancer cells. This occurs both directly and indirectly at the membrane and sub cellular levels. Several mechanisms including mitochondrial dysfunction, changes in cell membrane integrity and inhibition of DNA replication have been postulated as pathways through which cell death following hyperthermia occurs. The absorption of power by the tissue (SAR) and the destruction of cancer cell is highly dependent on time and the input power of the antenna. This paper presents the numerical analysis of the important parameters in microwave thermotherapy and the biological mechanisms involved in the death of tumour cells.