Silicon thin films were deposited by low-pressure chemical vapor deposition (LPCVD) on oxidized silicon substrates, from silane decomposition. The deposition temperatures used in our experiment have been 500, 530, 550, 590 and 615 degreesC and the pressure values were 20, 53 and 100 Pa. The microstructure and the surface roughness of as-deposited films were investigated by X-ray diffraction (XRD), spectroelipsometry (using the Bruggemann-Effective Medium Approximation with a multilayer model) and AFM techniques. Three different models, Tauc, Cody and Wemple-Di Domenico were used to estimate the values of the optical gap. The results show the influence of the microstructure on the physical and optical properties of as-deposited LPCVD silicon films. It is pointed out that polycrystalline silicon thin films can be obtained by the LPCVD technique below 550 degreesC.