The latest developed nanotwinned cubic boron nitride (nt-CBN) with isotropic nano-sized microstructure possesses an extremely high hardness (~100GPa Hv), very large fracture toughness (>12Mpa m<sup>1/2</sup>) and excellent high temperature stability. Thus nt-CBN is a promising tool material to realize ultra-precision cutting of hardened steel which is widely used in mold insert of optical and opto-electrical mass products. In view of its hard machinability, the machining mechanism is studied in this paper. Three feasible methods of mechanical lapping, laser machining as well as ion beam sputtering are applied to process nt-CBN. The results indicate that among the three kinds of methods, mechanical lapping not only can achieve the highest machining accuracy because of material removing at ductile mode completely, but also has satisfactory high material removal rate. Thus mechanical lapping method is appropriate to finish machining of nt-CBN cutting tool. Moreover, laser machining method can be only used in contour machining or rough machining of cutting tool as worse machined surface quality. With regard to ion beam sputtering method, the material remove rate is too low in spite of high machining accuracy. Additionally, no phase transition was found in any machining process of nt-CBN.