In this paper, amorphous, microcrystalline and polymorphous silicon films were prepared by plasma enhanced chemical deposition. Crystalline volume fraction of microcrystalline silicon was deduced from the Raman spectrum, and this fraction was validated using Bruggeman effective medium approximation (BEMA) model in spectroscopic ellipsometry measurement. The influence of thermal gradient on the deposition mechanism of microcrystalline and polymorphous silicon was investigated using a theoretical model. The dependence of crystalline volume fraction on film thickness shows there is a crystalline gradient between bottom and surface of microcrystalline film, and there is not such a gradient in polymorphous silicon film. Polymorphous and microcrystalline silicon have similar ordered state and density, which are signifieantly higher than those of amorphous silicon.