In this paper,the effect of substrate bias on the structural properti es of nanocrystalline silicon films deposited by hot-wire chemical vapor deposition (HWCVD) is studied by adding a direct current (DC) bias between the floated graphite substrate holder and the stainless steel chamber.T he structural properties of the films are analyzed by Raman,X-ray diffraction (XRD) and scanning electron microscop y (SEM).The results show that the crystalline fraction is increased from 42.2% to 46.2% by adding －300V subs trate bias.By further raising the substrate bias to －600V,the crystalline fraction is reduced to 40.6%.The su rface of the nanocrystalline silicon films deposited without and with －300V substrate bias is composed of crystall ites with length of about 200nm and width of about 100nm.While raising the substrate bias to －600V,the grai n size is obviously reduced,and a large number of fine grains appear in the deposited nanocrystalline silicon fi lm.These phenomena are mostly related to the electrons emitted from the hot wires.When the tantalum wires ar e heated to a relatively high temperature,a lot of electrons would be emitted from them.These emitted electr ons are accelerated in the electrical field,transferring the kinetic energy to the reactants by inelastic collisions.