1Cr11Ni2W2MoV is a new martensitic heat-resistant stainless steel utilized in the manufacturing of aero-engine high-temperature bearing components. Welding of this type of steel using fusion welding techniques causes many defects. Friction stir welding (FSW) is a valuable alternative. However, few investigations have been performed on the FSW of steels because of the high melting point and the costly tools. Numerical simulation in this regard is a cost-effective solution for the FSW of this steel in order to optimize the parameters and to reduce the number of experiments for obtaining high-quality joints. In this study, a 3D thermo-mechanical finite element model based on the Coupled Eulerian Lagrangian (CEL) approach was developed to study the FSW of 1Cr11Ni2W2MoV steel. Numerical results of metallurgical zones’ shape and weld appearance at different tool rotation rates of 250, 350, 450 and 550 rpm are in good agreement with the experimental results. The results revealed that the peak temperature, plastic strain, surface roughness and flash size increased with an increase in the tool rotation rate. Lack-of-fill defect was produced at the highest tool rotation rate of 650 rpm. Moreover, an asymmetrical stir zone was produced at a high tool rotation rate.