To control the nitriding effect, which is used to enhance the mechanical properties of surfaces, a fundamental understanding of this effect is required. Modern quantum-mechanical simulation methods make it almost impossible to perform cost effective and reliable studies on the mechanisms of the influence of nitrogen on surfaces. In this work, based on density functional theory calculations, the nitriding effect on the structure and mechanical properties of titanized steel was studied using a FeTi model. Two cases of the nitrogen presence in the Fe-Ti crystal are considered: uniform distribution and nitrogen clustering. Based on the formation energy calculations and the crystal orbital Hamilton population analysis, it is found that higher stability of FeTi is achieved at low concentrations of nitrogen up to 5.4% when nitrogen atoms are uniformly distributed, while upon clustering of nitrogen, FeTi becomes more stable at higher concentrations of nitrogen from 3.7% to 7.4%. The mechanical properties of nitrogen-containing FeTi suggest that Young's modulus and shear modulus increase with an increase of the concentration of nitrogen up to 5.4%. These findings not only deepen our fundamental understanding of the nitriding effect in titanized Fe-based steels but also offer valuable insights essential for carrying out an experimental study of various end products such as technical machine parts or medical implants, endowed with improved surface properties.