A series of experimental tests were carried out in this study to investigate the effect of tempering temperature on the metallurgical and mechanical properties of nitrided low-alloy steels. The low-alloy steel samples were previously subjected to tempering treatment. Phase constituents, hardness distribution, residual stresses, and growth rate of the nitrided layers are investigated using scanning electron microscopy, X-ray diffraction, Vickers microhardness, and mass spectrometry. It was found that the application of a tempering treatment at 600 °C before the nitriding process was accompanied by a high hardness and a distinct residual stress–depth profile inducing an improvement of the mechanical properties such as fatigue and wear resistances. Furthermore, to estimate the diffusion parameters and analyze their variation with the nitriding temperature, we proceeded in two steps. First, numerical simulations using the Abaqus finite element calculation code were carried out to calculate the nitrogen concentration and its diffusivity in the steel sample. Then, from these results, a mathematical approach was proposed to evaluate the diffusivity and the maximum nitrogen content.