In this paper we propose and demonstrate micro patterning processes of single crystal diamond using thermochemical reaction of diamond with a sidero-metal and elucidate the reaction involved. Single crystal diamond processes a variety of excellent characteristics, such as hardness and wear resistance, and hence, is expected to be a new material for not only micro machining tools but also innovative micro devices. It is mandatory to develop a patterning process of diamond with high precision and at low cost. Laser processing is currently widely used, but it is a serial process and costly. Film deposition and plasma etching are other effective methods while they are time consuming. Thermochemical reaction between diamond and sidero-metals is well known in the field of mechanical machining. Diamond tools cannot machine sidero-metals, such as iron, nickel, and cobalt, when the diamond tools wear instead of the sidero-metals. We used this reaction to micro pattern single crystal diamond. We used nickel as the sidero-metal that was patterned either directly on a bulk of single crystal diamond or on a silicon substrate. We term the former and the latter processes as direct and indirect patterning processes, respectively. In the indirect patterning a bulk of single crystal diamond was placed on the substrate. The pattern was negatively transferred to the diamond after thermal treatment in the air at ˜1000K in both processes. The sidero-metal layer can be patterned by photolithography, which enables precise manufacturing and mass production. The direct and indirect patterning achieved etching rates of ˜0.5μm/min and ˜0.2μm/min, respectively, both of which increased as the annealing temperature increased while the indirect patterning did not require micro patterning of photoresist on a bulk diamond several millimeters squire and involved much less difficult processes than the direct patterning. The thermochemical reaction is reported to be caused by; diffusion of carbon from diamond into the sidero-metal, oxidation-reduction reaction between diamond and the metal, oxidation of diamond, and carbide formation. Carbide formation was not observed when we used nickel as the sidero-metal. When the direct and indirect patterning was conducted in nitrogen, the pattern transfer was observed but the etching rate was extremely low. Therefore, oxidation-reduction reaction is dominant in the direct patterning. In the indirect patterning, the etching continued even after the nickel and diamond was not in contact. The thickness of removed diamond was by far greater than the thickness of the nickel layer. Hence, we consider that diamond was etched by oxide-reduction reaction between the diamond and the metal while they were in contact at the beginning and then, the oxidation of diamond became dominant in the indirect patterning. The process proposed herein is readily applicable to manufacture micro devices that exploit excellent characteristics of single crystal diamond.