The purpose of this study is to determine the dependence of the size of the thermal lesion on sonication time in an ex-vivo porcine liver sample during simulated High Intensity Focused Ultrasound (HIFU) tissue ablation. MRI guided HIFU ablations were performed on a freshly excised porcine liver sample at 70 W acoustic power. The size of the lesion (ablated zone) was measured at sonication times, 20 s, 30 s and 40 s. Numerical calculations were performed to validate the experimental lesion size. Also, a histology study of the ablated liver sample was performed to confirm cell necrosis within the ablated zones. It was found that the HIFU induced lesion size is strongly dependant on sonication time and lesion size almost doubles (increases from 0.368 to 0.715 cm2) as sonication time increases from 20s to 40s. The area of lesions, determined experimentally, agreed within 5% with results of numerical calculations.