Liver cancer is one of the most common leading causes of cancer death worldwide. Radio frequency ablation (RFA) is one of the best ways to treat liver cancer. It is very noticeable and surprising. The success of the treatment depends on choosing an appropriate condition for it. In the paper, the treatment of liver tumor by a needle electrode is studied theoretically. The liver tissue is considered as a cylinder with 120 mm in height and 50 mm in radius. The simulations are performed by solving Bioheat equation with considering the RF energy absorption in the tissue and tumor, the boundary and primary conditions. The modeling is done by the finite element method (FEM). Also, the effect of blood perfusion and the vessel are included in this study. The results at 10 minutes and electrical potential of 18 V show that an elliptical cancerous tumor with the large-diameter of 27 mm and the short-diameter of 9 mm is damaged completely. According the simulations, the needle RF electrode is suitable for the treatment of various dimensions, deep and metastatic cancerous tumors. Also, size, shape, and elongation of elliptical cancerous tumors which can be destroyed by using different electrical potentials, are estimated in the paper. The results show that increasing the electrical potential leads to the reduction of the elliptical elongation and the coagulation area will be closer to the sphere.