In this study, the effects of hardness on temperature increase of ST 37 steel during fatigue loading were investigated. Steel specimens are made of ST 37 steel and subjected to heat treatment to obtain different hardness. The specimens were subjected to reverse bending fatigue loading and the specimens were observed by using a infrared (IR) camera during the test. The obtained thermal images were recorded by FLIR E45 IR camera and then transferred to the image processing program developed by using MATLAB. Thus after image processing, thermal values used to detect the temperature rise of the surface of the steel specimen under fatigue loading were obtained. During the fatigue, the material is subjected to strain energy input which result in plastic or/and elastic deformation. This event results in an increase of temperature within material. The energy conservation requires that the generated heat shows itself as heat transfer by conduction, convection and radiation and internal energy increase. Besides, if the material has undergone plastic deformation, an additional term which accounts this effect should be included within energy conservation equation. In order to observe the effect of plastic deformation upon temperature increase of material, the ability of plastic deformation has been changed through the change of hardness and the thermal variations during fatigue has been investigated.