Ultra-fine diamond powders were prepared from C2H2 and H-2 by quenching a d.c are plasma let on a metal disc cooled with water. The powders obtained were agglomerated with very small particles about 20 nm in diameter, with an average size of about 80 nm. Raman spectra, X-ray diffraction, and transmission electron micrographs of them showed that each particle consisted of very small diamond crystals and a small amount of amorphous carbon, and that a single crystal particle had a twin plane. The formation mechanism of the powders has been briefly discussed from the viewpoint of the supersaturation ratio based on the classical theory of homogeneous nucleation.