Diamond single crystals are in great demand for applications in mechanics, optoelectronics and electronics. For the future industrial applications, the large-sized diamond single crystal with larger quantity and lower costs is inevitable. In our laboratory, we use hot-filament CVD (HF-CVD), which realizes synthesizing a diamond at lower costs. It is widely believed that the crystal growth rate of HF-CVD is very low for the synthesis of a large-sized diamond single crystal. In present work, our target is optimization of experimental conditions, for example, gaseous pressures and substrate temperature, for the synthesis with HF-CVD method of large-sized diamond single crystal. Diamond single crystals of over 100 pin in diameter were synthesized successfully with our method. Synthesized diamond particles were characterized by SEM and microscopic Raman spectroscopy in which laser light spot area size was I pm in diameter. Our final target is synthesizing diamond single crystal large enough in size so that they are useful in future electronic devices. (C) 2007 Elsevier B.V. All rights reserved.