Inconel 625 is a nickel-base supper alloy that is widely used in power plants industry, aerospace systems, and mineral industries due to its properties such as high tensile strength, high corrosion resistance and excellent fabricability. Resistance spot welding (RSW) is one of the important joining processes for assembling supper alloy sheets, because of accuracy and high production rates. In the present research, the influences of electrode tip diameter and other RSW parameters on distribution of temperature and nugget formation are investigated by the finite element method for Inconel 625 superalloy. The process is simulated with a 2D axisymmetric coupled electro-thermal and uncoupled mechanical finite element model by using ABAQUS software package. In order to improve accuracy of simulation, material properties including physical, thermal and mechanical properties is supposed to be temperature-dependent. The diameter of computed weld nuggets is compared with experimental results and good agreement is observed. So, FE model developed in this paper provides prediction of quality and shape of the weld nuggets and temperature distributions with variation of each process parameter, suitably. The results show that increasing electrode tip diameter decreases weld nugget diameter, in constant welding current, but in general, the electrode tip diameter cannot be selected less than a distinct value.