Fused deposition modelling (FDM) is one of the most practical additive manufacturing methods that produces parts in a layer by layer fusion by melting the polymeric solid filaments. Important parameters in polymer extrusion process in FDM are the entry velocity of the polymer filament into the extruder or entry flow rate, the temperature of the extruder and its geometry dimensions. Changing each one of these parameters affects the behavior of extruded molten polymer. In addition, it affects the final part quality and dimensional accuracy. The diameter of the molten polymer in the output of the extruder nozzle is increased due to the non-Newtonian behavior of the material. This phenomenon is called the die swell. The aim of this project is to investigate the ABS melt flow behavior in the extrusion process by experimental tests, mathematical modelling and finite element simulation. In the experimental tests, the rheological properties and viscosity models of the ABS polymer determined by the capillary rheometer test. Moreover, the die swell and the temperature field of the extruded polymer investigated experimentally. In the mathematical modelling, the effect of changing in the extruder geometry, flow rate and temperature on the extruder pressure drop was studied. In the finite element simulation, the polymer temperature field and the pressure changes distribution in the extruder, the amount of die swell, and the temperature field of extruded polymer were studied by the Ansys-Polyflow software. In this study, the parameters of the nozzle angle and tip diameter were considered at the values of 90 to 180 degrees and 0.2 to 0.5 mm, respectively. Also, the extruder temperature and its motor revolution for flow rate were considered at the values of 220 to 260 and 8 to 40 rpm, respectively. The results showed that by increasing flow rate, the ABS has less chance to reach the desired temperature at the center of the extruder. Moreover, with the angle increase and the tip diameter of the nozzle decrease, the pressure drop increases in the extruder. The nozzle angle of 90 degrees and the nozzle tip diameter of 0.3 mm provide the continuously melt flow in the extruder by a relatively high pressure drop. However, at the nozzle tip diameter of 0.2 mm the extruder pressure increases excessively. Furthermore, the extruder pressure and the die swell of the polymer decrease by rising the temperature and increase by rising the flow rate. The temperature of the extruded polymer at a certain distance from the nozzle has less drop by increasing the flow rate. According to the results of the changes in extruder pressure, die swell, and temperature field, for the hired FDM machine, in order to decrease the die swell, for improvement of the dimensional accuracy, and control the temperature drop of the extruded polymer, for improvement of the part strength, the flow rate of 2.077*10 -9 and the working temperature of 260 are desirable for the extrusion process. Keywords Fused Deposition modelling, Extrusion, ABS polymer, Mathematical modelling, Finite element simulation, Pressure drop, Die swell, Temperature field.