The SrTiO3 Thin films were deposited on Pt/SiO2/Si by ECR-PEMOCVD and their dielectric properties were investigated as a function of composition. The chemical bonding states of elements consisting these films were investigated using X-ray photoelectron spectroscopy (XPS) and Discrete Variational (DV)X alpha simulation. The asymmetry of composition dependence on dielectric properties was explained by the XPS data. Further, effectiveness of NH3 carrier gas over Ar to deposit SrTiO3 is discussed in detail. Based on these data, the thickness dependence of dielectric constant of BST films was explained.