With the purpose of reducing the tool wear in a diamond cutting operation of ferrous metals and improving the machined surface quality and machining precision, the tool wear mechanism was studied. Experiments of the frictional wear between diamond and steel were performed to imitate the tool wear process in a practical diamond cutting. The wear morphology of workpiece surface, the changes in chemical composition of work samples, and the transformation of crystal structure of diamond specimens were detected by the Scanning Electron Microscopy(SEM),X-ray Energy Spectrometer(EDS)and the Raman Spectroscopy(RS).Then , the graphitized degree of diamond was proposed as a criterion for assessing diamond wear in tests. The experimental results reveal that the wear of diamond relies heavily on mechanical forces and temperatures, and less on the sliding speeds and carbon contents of materials in the test. The wear mechanisms of diamond in this frictional wear experiment include graphitization, diffusion wear and oxidation wear, and the graphitization is found to be the dominant wear mechanism for diamond wear. In addition, the graphitized degree increases up to 83% with temperature rising of 15%. In conclusion,the tool wear mechanism should be considered comprehensively in condition of thermal-force coupling for further exploring technological measures with respect to reduce the tool wear.