A tool for fabricating micro/nanopatterns was utilized in space optics, virtual reality, augmented reality, and semiconductor industry. Nowadays, demand of manufacturing technique for ultra-precision is continuously increasing virtual reality and augmented reality industry across the board and core technique for manufacturing next generation lens is cutting tool fabricating technique with nanoscale. In particular, tools of micro/nanosize for ultra-precision machining was made by using an ultra-precision grinding, but it was difficult to fabricate tools which have under micro scale. Recently, results of studies with many researchers were pulsed laser ablation, electric discharge machining and precision grinding. However, previous studies are unsuitable in making tools of micro/nanoscale. Due to unique physical properties of diamond, it can be easily controlled by using focused ion beam. The surface properties of the diamond layer are affected because of the amorphous damage caused by the FIB gallium ions collision, implantation and these effects can make to be able to control the geometry of cutting tool. In this study, we carried out in fabricating diamond tools under micro scale by using FIB milling through various process studies and determined in order to optimize the length of unstable tool.