We report in-situ plasma diagnostics during the deposition of low dielectric constant SiOC(-H) thin films on p-Si(100) substrates by using plasma enhanced chemical vapor deposition with dimethyldimethoxysilane (DMDMS, C 4 H 12 O 2 Si) and oxygen gas as precursors. The bulk plasma was characterized by using Langmuir probe and optical emission spectroscopy as a function of radio frequency (rf) power. The electron density and electron temperature were found to increase with increasing rf power. As the applied rf power increases the -CH 3 groups present in the DMDMS precursor first dissociated into CH, C and O, without forming CH 2 and CH 3 radicals and then the CH radical further dissociated into C, H α , and H β related radicals, respectively. The SiO 2 -C 2 and SiO 3 -C, SiO-C 3 and C-C/C-H bonding configurations of the SiOC(-H) film were increased with increasing rf power whereas Si-O 2 bond decreased. These bonding structures help to reduce the k value of SiOC(-H) films. The lowest dielectric constant of 2.04 was obtained for the film with an rf power of 700 W.