The most common extrusion based technology in rapid prototyping is Fused Deposition Modeling (FDM). In FDM process, widely used materials are Acrylonitrile Butadiene Styrene (ABS) and Polycarbonate. In this study ABS-P430 material is considered. During the part build process, the rapid heating and cooling is happening on the build part which leads to high thermal gradient. This thermal gradient causes thermal stress; it will lead to deformation of build parts. In this paper a three dimensional transient thermo-mechanical Finite Element Analysis (FEA) had been used to find out the maximum principal stress and deformation of the build part. This FEA analysis is called as thermal and structural deformation model or 3D FEA model. In this model, the novel technique called Element birth/death is used in ANSYS11 to mimic the FDM process. The most influencing parameters of FDM process called orientation and layer thickness have been considered in a 3D FEA model to calculate the deformation of a part. To validate the work, a standard design which is considered in 3D FEA model is fabricated using dimension 1200es FDM machine using same orientation and layer thickness and deformation is measured. From the results it was observed that the relative error between 3D FEA model and actual fabricated model is found to be 3-6%. This 3D FEA model would be helpful for RP machine users to find the deformation of the build part before making the products. پرونده مقاله