The paper presents the results of the research of a laser surface hardening process applied to the C45E steel with graphite absorber coating on specimen surfaces. The evaluation of the laser surface hardening process was performed by measuring the IR radiation from the interaction spot. The results confirmeda strong correlation between the IR radiation voltage signal and the dimensions of microstructural changes occuring in the laser surface hardening steel. Changes of the absorbing coating thickness produced changes of the absorptivity of the laser beam, which was confirmed by measurements of the depth and width of hardened traces. For laser surface hardening the optimum coating thickness of a graphite absorber was determined. A factorial analysis and a method of orthogonal polynomials were used to determine response surfaces for the voltage signal of IR radiation the depth and width of the hardened trace achieved. Absorptivity difference between various graphite absorber coating thicknesses was evaluated based on the Ashby-Shercliff model.