In this study, the influences caused by the wear of grinding wheel are investigated experimentally. The infrared radiation pyrometer with an optical fiber is developed and applied for the temperature measurement in the surface grinding process of carbon steels. Experimental data such as the temperature, the number and the size of effective cutting grains and the grinding forces are obtained over the wheel life, and they are used for the calculation of the thermal energy fraction to the workpiece, the grinding wheel and the chips. Thermal damage begins to be caused on the ground surface, as the attrition wear of grains decreases the number of effective cutting grains and degrades the efficiency of grinding wheel. The measuring of radiation pulses from the cutting grains makes it possible to detect the degradation of grinding wheel at an early stage of wheel life and prevent the severe damages on work surface.