Carbon-doped oxide materials (SiCOH films) with k of 2.45 and good thermal stability at temperature 485℃ are prepared by electron cyclotron resonance chemical vapor deposition (ECR-CVD) from the mixture of decamethylcyclopentasioxane (D5, ［Si(CH3)2O］5) and methane (CH4). T he chemical structures of D5 as a liquid source and films deposited with differe nt methane flux are studied by Fourier transform infrared spectroscopy. The resu lts indicate that the increase of methane is of great advantage to the reservati on of ring structure of D5 as well as the formation of high density CHn fragments in films. The decrease of dielectric constant is induced by the com bined action of several factors: lower density of films caused by high density o f CHn fragments and formation of intrinsic pores, lower polarization caused by the formation of Si-C bond and decrease of-OH bonds.