Nanoscaled cones have been demonstrating their unique usefulness in diverse applications such as high-efficiency field emission, near-field optical microscopy, high-resolution atomic force microscopy, solar cells, optoelectronics and bio/chemical sensing devices [1][2]. Generally, the reactive ion etching of the substrates using patterned masks is one of the most promising techniques for the fabrication of nanocones. However, it can only be applied successfully to certain limited substrates [3][4].In this work, we report a significant subtractive formation process of large-area diamond conical nanostructure arrays on freestanding diamond films using a hot filament chemical vapor deposition (HFCVD) system with negative biasing of the substrates. The etching gas comprising of H2 and CH4 with typical flow rate ratios of 100 : (1.5-4) sccm was employed to generate the plasma at a chamber pressure of about 20-30 Torr. The temperatures of the Ta filament and substrate were set at 2000-2200 'C and 800-900°C, respectively, and the etching duration was 1-3 h. The etching effect of energetic ions on the formation of diamond cone arrays with controlled morphology has been studied in detail. The SEM image shown in Fig. 1 presented that the as-formed nanocones were of uniform cone angle of about 27°and a height of about 3 jtm; the cone density is about 5x107 cm-2. The TEM image of an individual cone indicated that an apex radius of about 1 nm is obtained as shown in Fig.2. The results also show that methylic ions dominantly contribute to diamond cone formation based on a neutral-ion charge exchange collision model. The self-organized selective sputtering process of as-formed hillock bottoms on roughened surface by low energetic ions plays key role for the formation and development of diamond cones [5].The field emission properties of diamond nanocone arrays were measured, which presented lower threshold voltage (6.5V/ it m) and higher emission current density (56014 A/cm2 at 12.5V/ 14 m) compared with those of planar diamond films (threshold voltage:10.8V/ ti m; current density: 60 V A/cm2 at 12.5V/ 14 m). Furthermore, field electron emission properties of an individual diamond cone are investigated using a home-made Double Probe Scanning Electron Microscopy (DPSEM) system. The as-formed sharp diamond cone exhibits high emission current of about 80 14 A at applied voltage of 100 V. The study has opened the possibility of using diamond cone arrays and individual diamond cone as display device and point electron source with excellent electron emission ability.