This paper describes the growth mechanism of silicon whisker on a silicon substrate using hot filament CVD reactor. Only hydrogen is used as source gas. The particle layer could be obtained at high filament current condition under hydrogen ambient. XPS analysis result suggests that the particle is composed of tungsten silicide. The deposition condition of the particle layer is much depended on the substrate size, surface condition and the distance between the substrate and the filament. The experimental results suggest that the silicon hydride, which generated at the silicon surface by hydrogen radical etching, react with the tungsten filament material around the filament, depositing on the silicon substrate. The silicon surface is etched by hydrogen radical and its resultant surface morphology is much depended on the particle deposition pattern. Many silicon whiskers, which diameter is varied from 10 to 50 nm, are observed on the textured silicon surface when the residence time of the source gas in the reactor is long. Each whisker has a silicon particle on their tip. The silicon hydride generated by the hydrogen radical etching is much absorbed to the silicide particle when the source gas residence time is long, enabling the silicon whisker growth from the particle. The results suggest that nm size whisker structure is much stable compare to the bulk silicon against etching reaction.