Silicon nanopowder that can be applied as a selective emitter in solar cells was prepared from the decomposition of silane (SiH4) gas by a radio-frequency (RF) thermal plasma. The RF thermal plasma offers a high-temperature and contamination-free environment to produce pure silicon material from SiH4. A steep temperature gradient in the thermal plasma enables the rapid quenching of decomposed material to synthesize nanosized particles. In this experiment, the input power of RF thermal plasma was controlled from 8 to 12 kW. The raw material of SiH4 gas was injected into an argon thermal plasma plume with argon carrier gas. Argon was also used as quenching gas to suppress the growth of silicon nanoparticles. In addition to the input power, the flow rates of carrier and quenching gases were employed as operating variables. The flow rates of carrier and quenching gases were controlled from 15 to 45 L min−1 and from 50 to 250 L min−1, respectively. Polycrystalline spherical silicon nanoparticles that were less than 100 nm in diameter were observed by transmission electron microscopy, Brunauer–Emmett–Teller (BET) surface area measurement, X-ray diffractometry, and Raman spectrometry. Among the operating variables, the flow rate of carrier gas showed the most significant effect on the size of the nanopowder. The smallest mean particle size of 36 nm was obtained from the highest carrier gas flow rate of 45 L min−1, because the growth of nanopowder was limited by the enhanced axial velocity and quenching rate of in-flight silicon nanopowder.