Low-k interlayer dielectric film with a relative dielectric constant (k) of 2.6 has been formed by plasma enhanced chemical vapor deposition (PE-CVD) with hexamethyldisiloxane (HMDSO), nitrous oxide, and dilution gases. The effect of the dilution gases on the vapor phase reaction, the deposition rate, the electrical property, the thermal stability, the structure, and the mechanical property of the PE-CVD low-k film were clarified. The vapor phase reaction in helium dilution gas hardly occurs compared with argon and nitrogen. However, the deposition rate using He dilution gas is smaller than that using either of the other two dilution gases. Ammonia ions were found to be contained in much greater quantities in the film deposited using N-2 dilution gas than in the film deposited using He and Ar dilution gases by thermal desorption spectroscopy. The pressure effect on the composition of the low-k film was also clarified. Hydrogen content in the low-k film increases with increasing deposition pressure.