An atmospheric pressure DC plasma torch reactor was employed using Hexamethyldisiloxane (HMDSO) and oxygen reactants in nitrogen carrier gas to produce silicon oxide nanoparticles and nanoparticle agglomerates with the diameters in the ranges of 8-14 nm and 130-260 nm, respectively. The mean sizes of primary nanoparticles and nanoparticle agglomerates as well as their surface properties are studied by varying the process conditions. In general the mean sizes of primary nanoparticles and nanoparticle agglomerates decrease with the increase of the plasma torch power, the increase of the oxygen concentration, and the decrease of the HMDSO concentration. Under those conditions, the more completely-oxidized silicon oxide clusters are formed and aggregate into the compact, densely-packed, and small-size primary silicon oxide nanoparticles, which further agglomerate into the densely-packed, small nanoparticle agglomerates. The hydrophilic and white nanoparticle powder with a low BET surface area can be thus obtained. Under the opposite processing conditions, the loosely-packed, low density and large-size carbon-containing silicon oxide nanoparticles and nanoparticle agglomerates can be obtained with hydrophobic surface, high BET surface areas, and gray color. (C) 2017 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.