The paper presents the results of the fatigue testing of heat-treated and thermochemically treated C75 steel with different process parameters in terms of working medium (gas, salt bath), temperature, and time. The experimental program aims to analyze the changes in microstructure under the influence of heat treatment and fatigue resistance. The relationships between the structural changes, the internal stress, and the heat-treated material's mechanical and physical properties can determine the first nano cracks leading to rupture propagation. Based on the experimental values of this paper, we highlight the dependence between the nature of the cracks and the stress to which the specimen was subjected. The paper presents a brief introduction to the fatigue test and the experimental tests performed to determine the fatigue resistance characteristics, the macroscopic analysis of the material, and the crystallographic analysis. The results obtained allow a comparison between the fatigue limits of heat-treated and thermochemically treated C75 steel in gas and salt baths.