The microstructure and its annealing behaviours of a Si∶O∶H film prepared by PECVD are investigated in detail using micro Raman spectroscopy, X-ray photoelectron spectroscopy and Infrared absorption spectroscopy. The results indicate that the as deposited a-Si∶O∶H film is structural inhomogeneous, with Si-riched phases surrounded by O-riched phases. The Si-riched phases are found to be nonhydrogenated amorphous silicon (a-Si) clusters, and the O riched phases SiOx∶H ( x ≈1.35) are formed by random bonding of Si, O and H atoms. By high temperature annealing at 1150℃, the SiOx∶H ( x ≈1.35) matrix is shown to be transformed into SiO2 and SiOx (x ≈0.64), during which all of the hydrogen atoms in the film escape and some of silicon atoms are separated from the SiOx∶H ( x ≈1.35) matrix;The separated silicon atoms are found to be participated in the nucleation and growth processes of solid phase crystallization of the a Si-clusters, nano-crystalline silicon (nc-Si) is then formed. The microstructure of the annealed film is thereby described with a multi shell model, in which the nc- Si clusters are embedded in SiOx (x =0.64) and SiO2. The former is located at the boundaries of the nc-Si clusters, with a thickness comparable with the scale of nc-Si clusters, and forms the transition oxide layer between the nc-Si and the SiO2 matrix.