[en] In this paper have been determined the effects of temperature distribution at the hottest fuel rod (Hot Fuel Pin) in the Bushehr nuclear power plant for defined thermal conductivities from different mathematical functions. Whereas Hot Fuel Pin is one of the most fuel rod from aspect of heat temperature distribution in core of nuclear power plant, thus in this article have been obtained the values of average temperature of fuel rod ( T_ave) and temperatures of inner and outer diameters of fuel rod (T_fi, T_fo) have been obtained truly for a cylindrical fuel. For calculation of temperature have been used two methods means: analytical and numerical methods and in the numerical method has been used from computer programming (Turbo Pascal). Moreover calculations of temperatures have been performed for two stages and results have been modified than previous stage and have been compared together. In every stage of calculations by two said methods, the values of thermal conductivity (k) have been obtained from two methods means: Lagrange and finite difference methods and their related mathematical functions. (Author)