Microcrystalline silicon thin films at different growth stages were prepared by hot wire chemical vapor deposition. Atomic force microscopy has been applied to investigate the evolution of surface topography of these films. According to the fractal analysis, it was found that, the growth of Si film deposited on glass substrate is the zero_diffused stochastic deposition; while for the film on Si substrate, it is the finite diffused deposition on the initial growth stage, and transforms to the zero_diffused stochastic deposition when the film thickness reaches a certain value. The film thickness dependence of island density shows that a maximum of island density appears at the critical film thickness for both substrates. The data of Raman spectra approve that, on the glass substrate, the a_Si:H/μc_Si:H transition is related to the critical film thickness. Different substrate materials directly affect the surface diffusion ability of radicals, resu lting in the difference of growth modes on the earlier growth stage.