This paper describes effects of intermittent Ar/CH4 quenching gas (QG) injection on the size and composition of Si/C nanoparticles synthesized using pulse-modulated induction thermal plasma (PMITP). Time-controlled feeding of feedstock (TCFF), with synchronous and intermittent injection of silicon feedstock powder to the PMITP, was used for high-rate production of Si nanoparticles. Also, Ar QG was supplied intermittently from the chamber wall to enhance the cooling effect further. The QG also included CH4 as a carbon source gas for Si/C nanoparticle synthesis. Intermittent QG injection timing was studied for the composition of Si/C nanoparticles. The synthesized particles were analysed using FE-SEM, XRD, TEM, EDS, and Raman spectroscopy. Furthermore, numerical thermofluid simulation was also conducted to obtain the time varying temperature distribution in the reaction chamber, considering intermittent QG injection. From this numerical calculation, the dependence of the minimum temperature on the QG injection timing was found. The above experimental and numerical results indicate that carbon-coated Si nanoparticles can be synthesized when QG is injected at appropriate timing into the PMITP with temperatures of 1000–2000 K.