Microwave ablation (MWA) cures cancer by inserting slender antennas into tumors and producing large amounts of heat to kill cancerous cells. A key challenge is the selective heating of cancer cells without damaging the surrounding tissue, which requires a control of heating power for appropriate temperature distribution. This study established a theoretical simulation model for the microwave ablation of a porcine liver by using a coaxial-slot antenna with 10 slots at high frequencies. The axisymmetric finite element method (FEM) with temperature dependent thermal and dielectric properties was used to describe the MWA distribution in the liver tissue by coupling the electromagnetic wave equations and the bio-heat transfer equations. The results demonstrated that the increase of microwave frequency could improve the therapeutic effect of tumor significantly since it had less collateral damage, more concentrated heated region, and better material response than conventional microwave ablations. For a spherical tumor with 10 mm of radius, the 12 GHz MWA reveals an optimum ablation performance with 21.0% of the collateral damage at the radius direction, and 5.5% of the collateral damage at the axial direction, respectively. The results show that the 12 GHz MWA produces more concentrated heat, causes the greatest difference in temperature-rise between the tumor tissues and the healthy tissues, and significantly reduces the over-treatment region for spherical tumor.