The purpose of this paper was to prepare ultrafine particles of Si-based intermetallic compounds, TiSi2, MoSi2 and VSi2, by an are plasma method with hydrogen addition. The properties of the prepared particles were affected by the vapor pressure ratio (Ti/Si, V/Si: 10(-1); Mo/Si: 10(-3)) of the constituent metals. The vaporization and condensation rates of the constituent metals should be controlled to prepare intermetallic compound particles in the case of large different vapor pressures. The vaporization rate can be controlled by H-2 concentration in the are; for example, an increase in H-2 concentration leads to an increase in Ti fraction in the prepared particles in a Si-Ti system. The preparation of ultrafine particles of TiSi2 was most successful from the 60-wt.% Ti raw material with the 50%-H-2 arc. MoSi2 particles were prepared from the 85-wt.% Mo raw material with the 50%-H-2 arc. A single phase of VSi2 particles was prepared successfully from the 60-wt.% V raw material with the 50%-H-2 arc. Another purpose was to investigate the mechanism of vaporization enhancement of particular metals from a metal mixture by hydrogen in are plasmas. The vaporization enhancement was mainly attributed to the formation of intermediate products such as hydride and/or activity modification by hydrogen in molten metals.