Silicon nanoparticles (Si-NPs) were directly synthesized from a Si bar electrode via a solution plasma. In order to produce smaller Si-NPs, the effects of different electrolytes and applied voltages on the product were investigated in the experiments detailed in this paper. The results demonstrated that the use of an acidic solution of 0.1 M HCl or HNO3 produced Si-NPs without SiO2 formation. According to the transmission electron microscopy and electron energy-loss spectroscopy, the obtained Si-NPs contained both amorphous and polycrystalline Si particles, among which the smaller Si-NPs tended to be amorphous. When an alkaline solution of K2CO3 was used instead, amorphous SiO2 particles were synthesized owing to the corrosion of Si in the high-temperature environment. The pH values of KCl and KNO3 increased during electrolysis, and the products were partially oxidized in the alkaline solutions. The particle size increased with an increasing applied voltage because the excitation temperature of the plasma increased.