The effects of the synthesis conditions on the size and morphology of silicon dioxide particles prepared in a plasma field were investigated systematically. In this study, silicon dioxide particles were synthesized by a plasma reaction of a mixture of tetraethylorthosilicate and oxygen in a coaxial-type nonequilibrium plasma reactor. Since particles suspended in a nonequilibrium plasma field are known to be charged unipolarly (negatively), the generation of agglomerated particles is expected to be suppressed. An increase in the rf power not only accelerated the plasma reaction, but also enhanced the formation of agglomerated particles. This is considered to be caused by the generation of smaller primary particles with increasing rf power, which led to insufficient particle charging in the plasma. The diameter of the synthesized particles decreased with decreasing residence time. In this case, nonagglomerated particles with a size of less than 10 nut could be produced, in contrast to those in the case of increased rf power. The effects of the precursor concentration on the particle characteristics were also examined. An increase in the precursor concentration resulted in smaller primary particles. In this case, the coagulation between small, and thus insufficiently charged, primary particles were considered to be predominant. Under selected operating conditions, nonagglomerated particles with a diameter of up to 50 nm could be prepared.