In the current study, a numerical model based on Lagrangian incremental formulation was developed for friction stir process (FSP) of nylon 6 and nylon 6/multiwalled carbon nanotube (MWCNT) composite to investigate the thermomechanical behavior of them (e.g. temperature, effective plastic strain distribution, material flow, and material velocity). For this purpose, FSP was used to disperse the MWCNTs among the polymer matrix of nylon 6, homogenously. The X-ray diffraction and scanning electron microscopy were used to investigate the properties of fabricated nanocomposite. The obtained results were then used to verify the objectives of numerical model. According to the results, the MWCNTs are separated from each other while passing the tool-pin due to high plastic strain applied on them and greater amount of the material accumulates in the AS of stirred zone after passing the tool-pin. Also, MWCNTs were homogeneously and straightly dispersed throughout the nylon 6. The straight and non-curved dispersion of MWCNTs results in growth of its reinforcement capability.