We studied the dielectric characteristics of low-k SiOCH thin films by Ellipsometry. The SiOCH thin films were prepared by deposition of BTMSM precursors on p-Si wafer by CCP-PECVD method. The nano-porous structural organic/inorganic hybrid-type of SiOCH thin films correlated directly to the formation of low dielectrics close to pore(k=1). The structural groups including highly dense pores in SiOCH thin films originated the anisotropic geometry type of network structure directing to complex refractive characteristics of SiOCH single layer on the p-Si wafer. The linearly polarized beam of Xe-ramp in the range from 190 nm to 2100 nm introduced to the surface of SiOCH thin film, and the reflected beam was Elliptically polarized by complex refractive coefficients of SiOCH dipole groups. The amplitude variation $psi$ and phase variation $Delta$ of the relative reflective coefficients between perpendicular and parallel components to the incident plane were measured by Ellipsometry. The complex optical constants n and k as well as the dielectric constant and thickness of SiOCH thin films were driven by the measured value of $psi$ and $Delta$.