Thermal ablation is conducted by utilizing temperature changes to destroy the abnormal tissue or restore its function. The latest technique in thermal ablation is Microwave Ablation (MW Ablation) that rely on the propagation of electromagnetic waves that is able to increase the temperature of a tissue rapidly. Changes in temperature during the ablation process will reduce the water content in the body tissue (78% of the liver tissue is composed of water) and affect the value of permittivity and conductivity of the tissue. These changes cause the impedance matching of the antenna applicator also change during the ablation process. The change on impedance will make the system does not match with the previous resonant frequency. In addition, the microwave ablation applicator, which is designed for specific frequencies (2.4 GHz and 915MHz) with narrow band characteristics will be suffered from aforementioned phenomenon. Meanwhile, according to the previous studies there are other prominent resonant frequencies that used for MW Ablation, i.e. 1.9 GHz, 6 GHz, 9.2 GHz and 10 GHz. Therefore to accommodate some of the frequencies that are widely used in microwave ablation technique and to tackle the problems arising from the reduced transmission efficiency, then an applicator that has ultrawideband (UWB) characteristics is proposed. In this paper, we design an applicator, which has the characteristics of UWB by modifying the ground plane of the applicator by a hexagonal-sectioned coil. The applicator is designed on a FR4 substrate and numerically simulated with liver phantom by using CST Microwave Studio. The proposed applicator is able to have wide bandwidth by 8.1 GHz, with lower frequency (fl) 1.84 GHz and higher frequency (fh) 10 GHz.