Thermoforming is widely employed in industry for the manufacture of light weight, thin walled prouducts from thermoplastic polymeric sheets. This process is used for industrial products including signage, housings, and hot tubs. It also produces much of the packaging in use today including blister packs, egg cartons, and food storage containers. This process has many advantages over other methods of producing these products, but also it has some limitations. The most important limitation or weakness of this process is nonuniform thickness distribution of the final product. To overcome this limitation various methods have been developed such as using plug molds or prestretching the sheet before forming. Controlling the process parameters is another way to achive a uniform thickness distribution. In thermoforming, a heated thermoplastic sheet is stretched into the desired shape by applying vaccum, pressure or mechanical load. In this process producing a part with a good quality and minimal time and cost is the objective of producers. For this perpose manufacturers have used trial and error methods to reduce costs and improve products but these methods were costly and time consuming. Over many years attempts have been made to simulate thermoforming process and thereby exploit modern computational tools for process optimisation. However, progress in this area has been greatly hampered by insufficient knowledge of the response of polymer materials under thermoforming conditions and an inability to measure this and other processing phenomena accurately. In this research modelling of thermoforming process of a rectangular polymeric specimen has been carried out with the aim of obtaining a suitable thickness distribution. Also effects of process parameters on the thickness distribution of the final product have been investigated. For this purpose; ABAQUS simulation software was used and constitutive equation of Maxwell viscoelastic model was adopted for describing the behavior of high impact polystyrene (HIPS) thermoplastic polymer during the forming process and the constitutive equation was implemented in user subroutine VUMAT. Shell elements were used for the discretization of the sheet because sheet thickness is much less than other dimensions. A quarter of the sheet and mold was modeled in the software for the sake of symmetry. The verification of the subroutine has been carried out in two ways. First pure shear test at constant strain rate on a shell element with Maxwell viscoelastic model for material behavior was simulated and simulation results were compared with experimental data. Secondly simulation of one the ABAQUS benchmark problems was carried out with the user subroutine and results were compared with exsisted results. Finally simulation of the thermoforming process of the rectangular specimen in air slip forming process was considered and Thickness distribution of the final product along the symmetric planes was investigated. Simulation results were in good agreement with experimental results. Effects of friction coefficient, plug velocity, sheet temperature and sheet dimension on the final part thickness was investigated. By increasing friction coefficient, plug speed, sheet dimension and decreasing sheet temperature part thickness increases. Friction coefficient has an important effect on the thickness distribution of the final product. Keywords: Thermoforming, Polymer, ABAQUS, Viscoelastic materials, User subroutines