The article presents the findings of a study on the machining of 10 mm thick Hardox 400 steel plates using the CO2laser. The purpose of the investigation was to investigate the relationship between the entropy and the hardness of machined surfaces. For this purpose, a new mathematical model is established to estimate the entropy, and its influence on the hardness is determined. The mathematical model is statistically and experimentally validated. An entropy variation ΔS = −330 mJ/K between 2 K is found, causing a decrease in hardness compared to the standard value. The influences of input parameters (laser power, cutting speed, and auxiliary gas pressure) on hardness are determined. It is demonstrated that the surface hardness is strongly influenced by the auxiliary gas pressure. The combination of laser powerP= 4200 W with gas pressurep= 0.45 bar at average cutting speedv= 1400 mm/min leads to a hardness of 38 HRC, extending the life and wear resistance of the cut parts.