To evaluate the performance of microwave ablation with an internally cooled antenna in ex vivo and in vivo porcine livers.Microwave ablations were performed in ex vivo and in vivo porcine livers with a 1.9mm-diameter 2450MHz microwave needle antenna. Four power settings (50, 60, 70 and 80W) were used in this study, while application time was fixed at 10min. Diameters of the coagulation zone were observed on gross specimens.Excessive elongation of coagulation along the needle shaft was not encountered in all ablations. An arrow-shaped charring was observed. As the power increases, the long-axis coagulation diameter for in vivo liver increased significantly (P<.05). The short-axis coagulation diameter for in vivo liver was significantly smaller than that for ex vivo liver (P<.05) but not statistically different among the four power settings. After 10min of ablation at 80W, the short-axis and long-axis coagulation diameter for in vivo liver was 4.92±0.15cm and 2.37±0.10cm, respectively.The internally cooled microwave antenna may be advantageous to minimize collateral damages.