Amorphous SiCOF films with high carbon concentration are prepared by PECVD (plasma-enhanced CVD) with TEOS/C4F8/O2. The dielectric constant of (α-SiCOF film is reduced to 2.6 and other electric properties are improved remarkably. The moisture resistibility of the film is also improved. Through FTIR and XPS analyses, the chemical construction of α-SiCOF film is investigated. The mechanism of improvements in electrical properties and stability in moisture is further discussed. It is found that the ionic polarization and orientational polarization decrease in α-SiCOF films and contribute a lot to the reduction in dielectric constant. In addition, because of the hydrophobicity of incorporated C−F bonds, the moisture resistibility of α-SiCOF film is improved.