Abstarct: The study of fluids flow features in sudden expansions is important from two aspects. From one side, this kind of flow has many industrial applications such as fluid output processes, filling the casting molds, heat exchangers, mextal forming, etc. From the other side, because of its almost simple geometry, it is used for estimating numerical methods and investigating features of flow like the size and intensity of the vortices. In this study flow and heat transfer of viscoelastic fluid in an axisymmetric sudden expansion with expansion ratio 1:3 is studied numerically for the first time. Most of the researches which have been done in this field focus on the flow of viscoelastic fluids, so due to the lack of a comprehensive research on the heat transfer of viscoelastic fluids flow in sudden expansions, conducting the current study seems to be necessary. Considering some of the thermodynamic and rheological properties of viscoelastic fluid as function of temperature is one of the other innovations of present study and due to the sensitivity of some of the viscoelastic fluid properties to temperature, considering this hypothesis seems to be essential. In this study, the open source software OpenFOAM that is a computational fluids dynamic (CFD) tool box is used for simulating flow and heat transfer of viscoelastic fluid. First of all, Non-Newtonian fluids are introduced and some subject matter regarding constitutive equations of viscoelastic fluid is provided. In the following, the general form of governing equations of flow and heat transfer including continuity, momentum and energy is expressed in cylindrical coordination and then, the general relations of viscoelastic fluid structural equation (EPTT model) and viscosity viscometric functions, are brought. After that, the numerical method that is used in the present study is described. In the current research, for explicit discretization of governing equations finite volume method is used. Also, for natural time gauging solution PISO algorithm is used so that flow and heat transfer parameters can stay constant and steady and by increasing time step and convergence of the parameters current and logical answers can be achieved. For validating numerical solution results of the flow and temperature field, analytical and numerical results of other references is used. At the end, obtained results from numerical solution of flow and heat transfer are presented in form of stream lines, flow and temperature contours, local nusselt distribution and etc. In the last chapter, the obtained results from the investigation are reported.