In recent years, utilization of polymers in various industries is caused to invention of new methods to produce new products. Thermoforming is one of the approaches to form polymer especially in home applinaces and packaging industries. In this process pre-extruded polymeric sheet is heated and formed to the desired shape. One of the problems in this process is to reach to the desired part dimensional accuracy of designed product especially when the part should be assembled in a base. Today, try and error method which is costly and time-consuming is a suitable approach to consider the effect of different process parameters on dimensional accuracy of products in both vacuum forming and drape forming processes. In this study, the final product is a cup with defined geometry and process parameters were initial sheet thickness, initial temperature and vacuum pressure for vacuum forming. For the drape forming process, in addition to the above parameters, plug speed in pre-stretching phase was also an additional parameter. The raw high impact polystyrene (HIPS) material used in experimental work had 0.15, 0.3 and 0.5 mm thickness. The experimental results showed that an increase in temperature and vacuum pressure will improve the part dimensional accuracy. The 0.3 mm sheet in thickness using vacuum forming and 0.5 mm sheet in thickness using drape forming give the best dimensional accuracy than other sheets. Decrease of the polymer viscosity due to increase the strain rate in drape forming process will increase the profile shape quality. Experimental results illustrated that in vacuum forming process stretches are biaxial while in drape forming process stretches are uniaxial. Comparison between vacuum forming and drape forming showed that vacuum forming is better than drape forming to reach the desired part dimensional accuracy. Also a hyper-viscoelastic material model is used to simulate the behavior of polymer in this study. Comparison between experimental tests and simulation results showed that the maximum deviation is about 29 % which indicates that hyper-viscoelastic material model could properly model thermo-mechanical behavior of HIPS. In addition a numerical study of friction coefficient revealed that decreasing the friction coefficient causes a better dimensional accuracy of the part. Also, a numerical study of the plug speed illustrated that the dimensional accuracy of the part will improve with an increase in the plug speed Keywords: Thermoforming, Finite element simulation, Viscoelastic, Hyperelastic, HIPS, Final profile shape