We investigated the decomposition process of tetramethoxysilane molecules in rf PECVD by a quadrapole mass spectrometer. There were discrete peaks in the fragmentation pattern of the reactant molecule in the m/e region of 60-152. On the other hand these peaks decreased drastically in the mass spectrum measured during deposition. Most reactant molecules were broken into small fragments in rf plasma. We conclude that the -CH3 groups contained in the deposited films originated from the CHn radicals adsorbed on the growing surface. We also discuss how to reduce the -CH3 groups in silicon-oxide films deposited at low temperature.